The available toxicological data for 1,3-butadiene are limited and contradictory. Three month toxicity and two year carcinogenicity studies have therefore been initiated to identify any potential hazard to occupationally exposed personnel. The results of the 3 month study are reported in this paper. Five groups of Sprague-Dawley rats were exposed to 1,3-butadiene gas at atmospheric concentrations of 0, 1000, 2000, 4000 and 8000 ppm v/v respectively, 6 hours/day, 5 days/week for 13 weeks. Forty male and forty female animals were used in each group, of which 10 of each sex were killed at 2 and 6 weeks. Exposure took place in 5 chambers, in an area prepared specifically for the handling of a potentially explosive gas-air mixture at concentrations just below the lower explosive limit. No untoward effects attributable to exposure were observed, except a moderately increased salivation, particularly in female animals during the last 5--8 weeks of exposure, at higher concentrations of butadiene. No effects considered to be attributable to treatment were seen in growth rate, food consumption, hematological and blood biochemical investigations or urine analysis after 2, 6 and 13 weeks exposure. Macroscopic and histopathological examination after 2, 6 and 13 weeks exposure showed no untoward changes related to exposure to butadiene gas. Erythrocyte and brain cholinesterase, erythrocyte osmotic fragility and neutrophil phagocytosis were also unaffected by treatment.