It has been shown that enhanced cholinergic tone induced by pyridostigmine (PD) increases both basal and GHRH-stimulated GH levels in both adults and children. In this study the effects of PD (60 mg orally) on GH secretion were studied both in the morning (from 8.00 to 12.00) and in the night (from 23.00 to 3.00) in 7 short children previously shown as having a normal spontaneous nocturnal GH secretion. In the morning, PD induced a GH increase higher than saline (peak, mean +/- SEM: 17.4 +/- 3.4 vs. 5.5 +/- 3.0 ng/ml, p less than 0.02; area under curve (AUC): 360.8 +/- 71.4 vs. 109.4 +/- 44.7 ng/ml/h, p less than 0.01). In the night, no difference was observed between GH secretion after PD (peak: 16.7 +/- 2.4 ng/ml; AUC: 468.2 +/- 95.5 ng/ml/h) and saline (peak: 16.0 +/- 2.7 ng/ml; AUC: 409.1 +/- 97.7 ng/ml/h). Spontaneous GH secretion was higher during the night than in the morning (p less than 0.02) whereas nocturnal GH secretion overlapped with that in the morning after PD. The ability of PD to increase GH secretion during the morning but not GH hypersecretion occurring at night implies that the cholinergic tone in the central nervous system areas controlling GH secretion is already maximally stimulated at night. Since, reportedly, the cholinergic system negatively modulates somatostatin secretion, presence of a physiologically reduced somatostatinergic tone may be envisaged at night.