Previous studies have shown that pyridostigmine (PD) is capable of increasing the growth hormone (GH) response to GH-releasing hormone (GHRH) in young healthy subjects. In order to investigate the influence of age and sex on the PD potentiation of GHRH-induced GH release, we have studied the GH response to GHRH (50 µg i.v.) 1 h after oral administration of placebo or PD (60 mg) in 8 young healthy men (aged 19-28 years) and 8 age-matched young women (aged 18-25 years) during the follicular phase of the menstrual cycle, as well as in 8 postmenopausal women (aged 57-62 years) and 8 age-matched elderly men (aged 56-64 years). In the same subjects the effect of PD alone (60 mg p.o.) was also studied. Furthermore, in 6 postmenopausal women and 6 elderly men, the effect of a 30-mg PD oral dose on GH secretion and GH response to GHRH was evaluated with a similar protocol. The GH responses (mean ± SE) to GHRH + placebo were similar in young men (peak 20.1 ± 2 ng/ml, AUC 1,250 ± 113 ng/ml/min) and women (peak 29.3 ± 2.3 ng/ml, AUC 1,769 ± 305 ng/ml/min). PD 60 mg was capable of significantly increasing the GH response to GHRH in young men (peak 43.5 ± 5.1 ng/ml, AUC 3,734 ± 472 ng/ml/min, p < 0.005) but not in women (peak 39 ± 2.3 ng/ml, AUC 2,479 ± 205 ng/ml/min). The GH responses to GHRH + placebo were also similar in postmenopausal women (peak 6.2 ± 0.7 ng/ml, AUC 540 ± 80 ng/ml/min) and age-matched men (peak 11.3 ± 0.6 ng/ml, AUC 763 ± 73 ng/ml/min) although these responses were significantly decreased when compared to those observed in young individuals. PD 60 mg administration induced a significant increase in GH response to GHRH both in postmenopausal women (peak 27 ± 3.6 ng/ml, AUC 2,224 ± 251 ng/ml/min, p < 0.001) and elderly men (peak 49.7 ± 2.4 ng/ml, AUC 4,557 ± 263 ng/ml/min, p < 0.001). The GH increment, however, was greater in elderly men than in postmenopausal women (p < 0.005). In young men, elderly men and postmenopausal women, PD 60 mg potentiated the action of GHRH rather than merely being additive. A similar effect was observed when a 30-mg PD dose was preadministered in elderly men and women. Our data clearly demonstrate a marked influence of age and sex on the PD potentiation of the GHRH-stimulated GH secretion. PD significantly potentiated GH responses to GHRH in young men, elderly men and postmenopausal women but n