1. The acute effect of progesterone at a physiological dose (50 micrograms) was investigated behaviorally by measuring postural deviation of rats bearing a unilateral lesion of the entopeduncular nucleus and biochemically by in vivo microdialysis in the striatum. 2. Ovariectomized female and intact male rats displayed a significant increase of postural deviation to the lesioned side 30-65 min after the subcutaneous injection of progesterone with a return to control values thereafter. 3. In addition, rats of both sexes had increased dopamine (DA) in the first 20-40 min dialysate collection following the progesterone injection. The metabolites of DA dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the dialysate were also increased after the progesterone injection and HVA levels remained elevated for a longer time than DA, up to 120 min after progesterone. 4. Progesterone, at a physiological dose and independently of estrogens can therefore rapidly increase DA release in the striatum of rats of both sexes as revealed in vivo behaviorally and biochemically.