The febrile response, as part of the Jarisch-Herxheimer reaction, was studied in 52 patients with proven primary and secondary syphilis treated with penicillin. Temperature elevation was noted as early as 1 hour and as late as 11 hours after the initial injection. The largest number (about 46%) occurred within three to five hours. Peak temperature elevation occurred most frequently six hours after onset of therapy. Fever ranged from 37.7 C (99.8 F) to 40.0C (104.0 F) and subsided in all cases within 18 hours.