To clarify the behavioral characteristics of rats neonatally treated with 6-hydroxydopamine (6-OHDA), their performance on 4 aversive learning tasks, 3 active (shuttle, one-way, and rearing) avoidance tasks and one passive (step-through) avoidance task, was examined. On days 2 and 4 after birth, each rat of F344/Du strain received bilateral intraventricular injections of 6-OHDA (35 micrograms x 2) or vehicle solution following desmethylimipramine (20 mg/kg, s.c.) pretreatment. From day 90, each rat was trained in one of the 4 avoidance tasks. 6-OHDA-treated rats showed significantly less avoidance responses in the shuttle and the one-way avoidance tasks, but their performance on the rearing and the step-through passive avoidance tasks was not significantly different from that of control rats. The differential impairment of avoidance suggests that 6-OHDA treatment does not cause a general learning deficit, but facilitates rearing and/or jumping responses in aversive situations, which results in inappropriate escape responses.