In 59 workers between the ages of 22 and 66, who had been exposed to pesticides, and 31 healthy controls between the ages of 22 and 28, the hepatic demethylation capacity was studied using the aminophenazone breath test (ABT). The ABT was carried out in two versions (version A: 111 kBq 14C-aminophenazone per 630 mg, version B: 111 kBq 14C-aminophenazone per 3 mg). The amount of 14CO2 expired per mmol CO2 per 70 kg body weight (b.w.) detected 1 h after 14C-aminophenazone intake was used to determine the demethylation capacity of the liver. The amount of expired 14CO2 depended on the ingested dose (B greater than A). The 14CO2-values measured in 13 controls did not differ from those obtained in 37 subjects who had been exposed to pesticides for 650 h per year on the average [649 vs 726 DPM/mmol X 70 kg b.w. (A)]. The 14CO2-values obtained in 22 subjects exposed to pesticides for 990 h per year on the average (B) were lower than those obtained in 18 healthy controls (736 vs 1024 DPM/mmol CO2 X 70 kg; P less than 0.05). The 14CO2-values of ABT decreased with increasing length of exposure per year (B; r = -0.51, P less than 0.05).