We have recently reported that parkinsonian patients show a significant GH response to gamma-hydroxybutyric acid (GHB), but not to gamma-aminobutyric acid (GABA)-ergic drug administration. In order to establish whether muscarinic cholinergic receptors mediate the GH secretion induced by GHB, normal men and parkinsonian patients were tested with GHB both in the absence and in the presence of the anticholinergic agent, pirenzepine. Both normal controls and parkinsonian patients showed a significant serum GH rise in response to GHB (25 mg/kg body weight p.o.) even though a slightly, but significantly lower response was observed in parkinsonian patients. Pretreatment with pirenzepine (100 mg p.o. 2 h before GHB) completely suppressed the GHB-induced GH release in both normal controls and parkinsonian patients. These data indicate that a cholinergic mechanism mediates the GH response to GHB in normal men. In addition the data indicate that this pathway is preserved in the parkinsonian brain.