The high-affinity selective dopamine D2 agonist [3H]CV 205-502 was utilized to in vivo label brain dopamine D2 receptors in the rat. Intravenous administration of [3H]CV 205-502 resulted in a selective accumulation of radioactivity in the striatum and in the pituitary. Smaller amounts of binding were found in the hypothalamus and cortex and non-significant binding was seen in the cerebellum. The binding of [3H]CV 205-502 was stereospecifically blocked by (+)-butaclamol but not by (-)-butaclamol. In vivo binding of [3H]CV 205-502 was also dose-dependently blocked by other neuroleptics including sulpiride, haloperidol and spiroperidol, dopamine agonists such as bromocriptin, apomorphine and C1 201-678 but not by dopamine D1 antagonists or serotonin-2 antagonists. The regional distribution of the sites labeled in vivo by [3H]CV 205-502 was investigated by autoradiography and compared with the distribution seen after in vitro labeling in the consecutive sections, following washing of the label. The autoradiograms reveal the labeling of the same areas seen when in vitro autoradiography was used. High densities of binding were localized in the nucleus accumbens, striatum and olfactory tubercle as well as in the olfactory bulb. Lower densities were seen in the substantia nigra pars compacta and in the ventral tegmental area as well as in the stratum lacunosum molecular of the hippocampus and in the superior colliculus. The intermediate lobe of the pituitary also presented high densities while in the anterior pituitary only intermediate densities of receptor binding were observed. These results demonstrate that [3H]CV 205-502 is the first high-affinity agonist useful for the autoradiographic visualization of dopamine D2 receptors after in vivo labeling. This compound could be modified for utilization in positron emission tomography imaging of dopamine D2 receptors in the living animal.