The 24-hr LD50 values of carbon disulfide (CS2) were estimated in 1-, 5-, 10-, 20-, 30-, and 40-day-old rats. CS2 was least toxic to 20-day-old rats (LD50 1545 mg/kg, ip) and most toxic to 1-day-old rats (LD50 583 mg/kg, ip). Twenty-four hours after administration of CS2 (375 mg/kg, ip) to 1-, 5-, 10-, 20-, 30-, and 40-day-old rats, significant inhibition of cytochrome P-450 and aniline hydroxylation was observed in rats of all ages studied except the 1-day-old rats. Following incubation of hepatic microsomes isolated from 1-, 5-, 10-, 20-, 30-, and 40-day-old rats with CS2, decreases in activity of the hepatic mixed-function oxidase enzyme system and/or concentration of cytochrome P-450 were observed when an NADPH-generating system was present during incubation. When hepatic microsomes isolated from rats of all ages studied were incubated with C35S2, 35S was covalently bound to microsomal protein in the presence of an NADPH-generating system. Also, more 35S than 14C was covalently bound to microsomal membranes after incubation of microsomes isolated from rats of all ages studied with C35S2 or 14CS2 in the presence of an NADPH-generating system. The results of this research demonstrated the LD50 of CS2, the effects of CS2 on the hepatic mixed-function oxidase enzyme system, and that the conversion of CS2 to a covalently binding sulfur-containing biotransformation product varied with age in developing rats.