Temperature during growth had a strong influence on light-saturated rates of photosynthesis and respiration in rye (Secale cereale). Carbon dioxide uptake was depressed at the low temperatures and reached maximal values higher than 30 milligrams of carbon dioxide per square decimeter per hour at the intermediate temperatures. Respiration rates increased substantially while growth temperature decreased. Simazine treatment (0.00, 0.03, 0.08, 0.12 milligrams per liter) resulted in no significant stimulatory or inhibitory effects at low temperatures, but progressively inhibited growth and photosynthesis at the higher temperatures. Respiration rates were not significantly influenced. Thus, the effect of simazine is strongly temperature-dependent and is more pronounced on photosynthesis than respiration.