To compare the functions of the lateral olfactory tubercle (OT) and the medial nucleus accumbens (ACC), dopamine (DA), (3,4-dihydroxyphenylimino)-2-imidazoline (DPI), and ergometrine were injected into the brain of rats familiarized with the experimental cage in which locomotor activity was assessed. In all tests a volume of 0.5 microliter per side was used. Both DA (1-10 micrograms) and apomorphine (1-10 micrograms) increased locomotor activity when injected into the OT; similar injections into the ACC produced inconsistent effects. The OT effects were short-lasting, dose-dependent and antagonized by haloperidol (0.5-2.5 micrograms) in a dose-dependent manner. DPI (1-10 micrograms) too produced an increase when injected into the OT; this response was long-lasting, dose-dependent and potentiated by ergometrine (0.1-1.0 microgram). Ergometrine (0.1-1.0 microgram) dose-dependently increased activity over a period of 200 min in ACC and OT rats, although the response in OT rats was much smaller than that in ACC rats. Only the ergometrine response in ACC rats was dose-dependently suppressed by DPI (1-10 micrograms). ACC rats tested during the light period showed a weak stimulatory response to ergometrine in comparison with ACC rats tested during the dark period; OT rats showed reversed light/dark periodicity. Thus, OT rats significantly differed from ACC rats with respect to locomotor responses to dopaminergic agents, their pharmacological profile and their light/dark periodicity. Evidence is provided that the lateral tuberculum, but not the medial accumbens, is responsible for the stimulatory effect of dopamine and related compounds.