Adult male Sprague-Dawley rats were administered dopamine (DA) or l-noradrenaline (l-NA) locally into the nucleus accumbens or in the neostriatum. Six minutes following the injections the animals were placed in an open field arena (700 X 700 mm) and their locomotor activity was recorded every 3 min for maximally 60 min. In the nucleus accumbens both DA (10-160 micrograms/side) and l-NA (2.5-40 micrograms/side) produced a suppression of the initial (0-3 min) exploratory locomotor activity in the open field arena. The highest doses of the respective drug, 80-160 micrograms of DA and 20-40 micrograms of l-NA, produced stimulation of the locomotor activity at a later time interval (6-9 min). The number of rearings during the initial exploration (0-3 min) was suppressed by DA (10-160 micrograms/side) as well as by l-NA (2.5-40 micrograms/side). When administered to the neostriatum, DA (80-160 micrograms/side) produced a stimulation of locomotor activity, 6-9 min after placement in the open field. The administration of l-NA (20-80 micrograms/side) to the neostriatum produced a suppression of the exploratory locomotor activity (0-6 min). The number of rearings were reduced by the administration of l-NA (20-80 micrograms/side) whereas no significant effect was observed after the administration of DA (5-160 micrograms/side). As assessed in the present experiments DA and l-NA produced identical effects in the nucleus accumbens, l-NA being about 4 times as potent as DA, whereas opposite effects were produced by l-NA and DA when applied to the neostriatum.