The effects of halothane on deoxyribonucleic acid (DNA) synthesis and events preceding DNA synthesis have been examined in Chinese hamster fibroblasts in culture. DNA synthesis was studied by the uptake of3H-thymidine during short periods of incubation that minimized effects on cells in the presynthetic phase (G 1). Halothane produced slight but significant dose-related depression of3H-thymidine uptake (20 per cent depression with 2 per cent halothane). In a separate series of experiments. synchronized cultures were exposed to 1–3 per cent halothane in G 1 phase for three or five hours. Halothane caused a postponement of onset of DNA synthesis (S phase), indicating a delay in G 1. This delay roughly equalled the duration of exposure to 3 per cent halothane but was less with 2 per cent halothane. The delay was only about one hour with 1 per cent halothane.