Induction of hepatic ornithine-decarboxylase (ODC) activity by 4,4'- methylene-bis(2-chloroaniline) (101144) (MOCA) was studied in rats. Male Sprague-Dawley-rats were injected intraperitoneally (ip) once or twice with 0 or 75mg/kg MOCA or once with 0, 15, or 75mg/kg MOCA orally or ip. They were killed 6, 12, 18, 24, 42, or 48 hours after the last dose. Liver cytosol was isolated and assayed for ODC. Maximum induction of ODC activity after a single ip dose of 75mg/kg MOCA occurred 18 hours after injection; activity returned to the control value 42 hours after dosing. In rats that received two 75mg/kg doses, the extent of ODC induction after the second dose was similar to that of the first dose. MOCA at 15mg/kg did not significantly increase ODC activity. Oral induction of ODC activity by 75mg/kg MOCA was less than ip induction but significantly greater than the control value at 18 hours. The authors conclude that MOCA does not cause a prolonged induction of hepatic ODC activity and that the activity is refractory to a second MOCA exposure.