The effects of bromocriptine on potentially dissociable functions were investigated in this double-blind placebo-controlled trial in previously untreated Parkinsonian patients. Bromocriptine-induced reductions in response time were independent of variations in the complexity of the cognitive or motor programming components of reaction time tasks. These results suggest that bromocriptime speeds up response initiation, and it may also improve some "early" visual processes. No clear-cut verbal memory, visuospatial or general intellectual changes were produced by bromocriptine; however, further evaluation of the effects of bromocriptine on "executive" cognitive functions is clearly required. Psychiatric distress was significantly reduced when the active drug was administered. While mood improvements could not be statistically dissociated from motor improvements, review of the literature suggested that decreased emotional distress may have arisen through bromocriptine's action on neural circuits modulating mood. Some clinical implications of bromocriptine's effect on mood are mentioned.