Male Long-Evans rats that had been exposed in utero to 5 or 8 mg/kg of methyl mercury administered as a single dose on either days 8 or 15 of gestation were tested as adults in two operant tasks. In one task the animals were trained on two-way avoidance to a criterion of 10 consecutive avoidances. Following acquisition the animals were extinguished and 24 hours later re-trained to the previous criterion. Animals treated with 8 mg/kg on day 8 of gestation required significantly more trials to reach criterion during reacquisition than controls. Rats treated on day 15 with either 5 or 8 mg/kg took significantly more trials to reach criterion during acquisition than controls, and of the 8 mg/kg group 55% failed to reach criterion. Rats treated with 8 mg/kg of mercury on day 8 of gestation acquired a DRL-10 sec task at the same rate as controls. When challenged with d-amphetamine the treated animals were less disrupted at the higher dose (1.0 mg/kg) than controls, suggesting a shift in the dose response curve for this psychoactive drug. Activity measures taken simultaneously with the DRL session confirmed this shift in amphetamine effect. Results suggest that a single prenatal exposure to methyl mercury can affect learning and drug sensitivity of the adult animal. Additionally, mercury exposure in late gestation has more deleterious consequences on learning ability than early exposure.