The effect of diazepam was tested on self-stimulation in 21 mice with a bipolar electrode in lateral hypothalamus (HL), dorsolateral hippocampus (HPC) or lateral entorninal cortex (CEL). Diazepam at 0.5, 1 and 2 mg/kg significantly increased self-stimulation rates in HL while 4 and 8 mg/kg had no significant effect. Similar increases were seen in CEL mice but high doses produced a significant suppression. HPC animals showed a drastic suppression of self-stimulation beginning at 2 mg/kg of diazepam, lower doses having no significant effect. The results demonstrate that entorhinal and hippocampal self-stimulation involve at least partly independent neuronal mechanisms and point to a possible inhibitory influence of HPC and CEL.