The influence of different pretreatments upon locomotor stimulation, induced by injection of ergometrine into the nucleus accumbens of rats, was investigated. The noradrenergic antagonists phenoxybenzamine and propranolol and the serotonin antagonist methysergide produced no clear changes. Reserpine, alone or in combination with alpha-MPT, considerably shortened the delay between injection of ergometrine and start of locomotor stimulation. Ro-DOPA, but not Ro-5-HTP, clearly antagonized the locomotor stimulation. The effect of ergometrine was strongly diminished following injection of haloperidol directly into the nucleus accumbens. A strong inhibition was also observedfollowing intracerebral administration of the imidazoline derivative (3,4-dihydroxy-phenylamino)-2-imidazoline (DPI), but not after injection of the structurally related compound clonidine. DPI by itself and also the ergot derivatives ergocornine, bromocryptine, LSD, dihydroergotamine and methysergide in doses 5--10 times as high as that of ergometrine failed to produce locomotor stimulation following injection into the nucleus accumbens. The results are discussed, especially with regard to the role of dopamine.