The toxicity of aminophylline and caffeine was studied in adult and 2-day-old rats following a single subcutaneous injection of the respective drug. Following the injection of high doses of either methylxanthine, adult rats developed convulsions, tremors, lethargy and licking of lips. In adult rats, the LD50 of caffeine and aminophylline was the same after 24 h and after 1 week of observation: caffeine 265 mg/kg, and aminophylline 202 mg/kg (theophylline base 172 mg/kg). In young rats, the LD50 was greater when the observation was carried out for 1 week than at 24 h after the injection; at 24 h: caffeine 220 mg/kg, and aminophylline 169 mg/kg (theophylline base 144 mg/kg); at 1 week: caffeine 155 mg/kg, and aminophylline 140 mg/kg (theophylline base 119 mg/kg). Young rats failed to gain weight at a normal rate after administration of either methylxanthine. The greater toxicity of both methylxanthines in newborn animals may be at least partly due to the extremely slow elimination of theophylline and caffeine in the neonate.