Pregnant female rats were anesthetized with halothane for two hours during the middle of either the first, second, or third trimester (each trimester in the rat is seven days). Adult male offspring of these exposed mothers and of unexposed controls were tested on a difficult visual discrimination task and assessed for sensitivity to electric footshock (painful stimulus). Measures of activity, water intake, and adult body weight were also taken. Although exposure to halothane produced no significant change in the latter three measures, offspring of mothers exposed in the first and second trimesters took 39 and 41 per cent more error trials, respectively, to learn the maze task. First- and second-trimester-exposed offspring also had 26 per cent lower footshock response thresholds to the highest magnitude of response. Offspring of mothers exposed in the third trimester were not significantly different from controls in any of the measures taken. The impairments seen in the first-trimester-exposed (day 3-blastula) offspring may have been due to residual halothane or halothane metabolites retained until later in pregnancy. The data indicate that the (possibly teratogenic) effects of exposure to halothane during early development in the rat are maximal during the second trimester, when organogenesis is occurring, and that exposure to halothane during this period produces learning deficits and changes in footshock sensitivity in adult offspring.