To throw light onto the mechanism(s) by which the cholinergic system influences growth hormone (GH) release, the effects of two muscarinic receptor blockers, pirenzepine and atropine, and of an acetylcholinesterase inhibitor, pyridostigmine bromide, on the GH response to GHRH-44 were studied in 19 normal volunteers. Moreover, the effects of pirenzepine administration on plasma GH levels both in basal conditions and after stimulation by GHRH-44 and TRH were studied in 9 acromegalics. Both pirenzepine (0.6 mg/kg i.v., 5 min before GHRH) and atropine (1 mg i.m., 15 min before GHRH) blunted the GH response to GHRH (1 µg/kg i.v. bolus) (area under the response curve, AUC: 81.3 ± 17.3 vs. 481.2 + 211.3 ng/ml/h for pirenzepine and 100.2 ± 27.0 vs. 364.7 + 81.0 ng/ml/h for atropine; p < 0.01). Pyridostigmine (120 mg orally, 30 min before GHRH) induced a variable but significant (p < 0.02) rise in basal plasma GH levels and, furthermore, an unequivocal potentiation of the GH response to GHRH (AUC: 1044.6 + 245.3 vs. 481.2 + 211.3 ng/ml/h; p<O.Ol). In all but one acromegalics 0.6 mg/kg i.v. pirenzepine was unable to modify the basal GH levels whilst it showed a variable inhibitory effect on the GH response to GHRH. The GH response to TRH (200 µg i.v. bolus) was instead unmodified by pirenzepine. In conclusion, muscarinic receptor blockade inhibits while cholinergic potentiation seems to positively modulate the GH response to GHRH. Therefore, the cholinergic system seems to positively modulate the GHRH effect on somatotrophs. In acromegalics pirenzepine is often able to inhibit the GH response to GHRH. On the contrary, the abnormal GH response to TRH seems independent of the cholinergic control.