The scene takes place in Jean's apartment. Jean lies in bed, coughing. Berenger comes to visit, knocks at the door, and an old man answers from a few doors down and says he thought he was looking for him, his name is also Jean. A disheveled Jean finally opens the door, wearing green pajamas. Both men make similar comments to each, asking why the other is not at the office and why the other's voice was unrecognizable through the door. Berenger apologizes for their argument the previous day. At first, Jean has no recollection at all about the rhinoceroses. Berenger smoothes things over by saying that it turns out both of them were right—there are both uni-corned and bi-corned rhinos.
Jean's voice grows more hoarse, and Berenger comments on this. Jean says it's Berenger's voice that has changed. Jean says his forehead hurts, though he can't remember knocking it on anything. Berenger suggests he did it in his sleep, during a dream, but Jean says he never dreams, that he's always in conscious control of his thoughts. Berenger points out a bump on Jean's nose, and Jean investigates it in the bathroom. When he emerges, his skin is greener. Berenger advises Jean to seek a doctor, but Jean, in an increasingly bitter mood, calls doctors quacks and says he trusts only veterinarians. They continue to quarrel as Berenger notices more odd signs—thickening skin, a hoarser voice, noisy breathing—and Jean grows more misanthropic, declaring that people disgust him and that he'll run them over if they don't step out of his way.
Jean paces the room like a caged animal and undoes his now-uncomfortable pajamas. He makes the preliminary stages of a growl. He runs to the bathroom to cool down and reemerges even greener and with a larger bump. Berenger informs him of Mr. Boeuf's transformation, and Jean doles out various explanations: Boeuf was fooling them in a disguise; he had a secret side that he never revealed; and turning into a rhino was a pleasurable exercise for him. He defends the rhino's right to live, which Berenger agrees to so long as they don't destroy human life. Jean proposes a return to the primal laws of nature instead of morality. He continues to move in and out of the bathroom, each time appearing more and more like a rhino and losing his human voice. He pronounces humanism dead and sheds his itchy clothes. He barrels his head down at Berenger, apologizes, and runs into the bathroom. Berenger is about to escape, but follows Jean into the bathroom, saying he can't leave his friend like this and he'll call Jean a doctor. Off-stage in the bathroom, Jean shouts that he'll trample Berenger, and a rowdy fight ensues. Berenger escapes and closes the bathroom door behind him (but is pierced by a rhino horn) as Jean, now a full-blown rhino, tries to break free.
Berenger alerts the tenants in the building to the rhino's presence in the building. The old man ignores his pleas and accuses Berenger of disturbing him and his wife. Berenger looks for the porter, but another rhino pops up in the porter's lodge. He returns to the old man's apartment, where two rhinos have replaced the old couple. Berenger runs back into Jean's apartment and looks out the window to the street, where a herd of rhinos march. Everywhere he turns, there are more rhinos. The bathroom door is on the verge of breaking. He throws himself against the wall and breaks through it. He runs through the street, yelling, "Rhinoceros!"
The most prominent feature of this scene is Jean's gradual transformation into a rhinoceros. Ionesco manages to make it plausible by having Jean disappear for moments into the bathroom, where he can alter his visage and body off-stage. His green pajamas serve a double use as a prop, foreshadowing his change in pigmentation and becoming a human nuisance to the emerging rhino. But the simplest of effects, and most powerful, is Jean's changing voice. The unique inflection of the individual voice is essential to humanity, and the subsequent loss of language seems nearly secondary to the ability to sound like a human.
Instead of parallel dialogue, a hallmark of the previous act, Ionesco deploys coincidence here as Jean and the old man share the same first name (not "Old Man," but "Jean"). This coincidence is further evidence of collective consciousness in that both men can be called Jean, and neither man distinguishes or affirms his human identity before turning into a rhino. The coincidence also attests to the increasing oddity of logic in the play. Berenger tries to make sense of the rhinos; he decides that it doesn't matter where the rhinos come from, but the "important thing, as I see it, is the fact that they're there at all, because ." He doesn't finish the sentence, which speaks volumes: the rhinoceroses are there both because there isn't a rational explanation, and also because absurd and apathetic humans don't take responsibility for making a life meaningful (or finishing a sentence, for that matter).