Male albino rats treated with 6-hydroxydopamine (6-OHDA) became more hyperactive than did vehicle-treated controls when both groups were water-deprived. Rats were treated with vehicle, 150 or 250 micrograms of 6-OHDA intraventricularly, after pretreatment with desmethylimpramine (25 mg/kg) and pargyline (50 mg/kg). Eleven days after these treatments, under ad libitum water conditions, the 6-OHDA-treated rats were slightly hypoactive. After several days of water-deprivation all three groups showed increased mean locomotor activity levels, but rats treated with 6-OHDA showed a much greater increase than did vehicle-treated rats. Under subsequent ad libitum and deprivation conditions, locomotor activity decreased and increased, respectively, in all 3 groups. Again, the changes in activity levels of the 6-OHDA-treated groups were greater than those of the vehicle-treated group. In addition, rats treated with 250 micrograms 6-OHDA seemed to become sensitized to the novel environment of the stabilimeter rather than habituating to it. Dopamine (DA) levels were decreased as a result of the injections of 6-OHDA, and significant correlations were found between DA levels in the caudate-putamen and nucleus accumbens, and locomotor activity levels. These results, as well as those obtained by others, suggest that there is an interaction among DA levels, deprivational states, and locomotor activity levels in rats.