We measured regional cerebral blood flow (CBF) with single photon emission computed tomography and Xenon 133 in 8 patients with Parkinson's disease (stages I or II) before and after acute oral administration of 10 mg bromocriptine. Moreover, Columbia University Rating scale and neuropsychological tests were assessed before and after drug administration. Bromocriptine induced a significant (p less than 0.01) increase in CBF (+ 12 p. 100) in all the regions studied unrelated to pCO2 variations. The Columbia motor scale score decreased (by 16 p. 100) whereas those on the neuropsychological tests did not vary. There was no relationship between the improvement in motor performance and the increase in CBF. The results suggest that the effect of bromocriptine on CBF is not mediated by an action on the major dopaminergic pathways in the brain but rather by a direct effect on dopaminergic receptors located on cerebral vessels.