To explore the effect of estradiol and progesterone on the GH response to the indirect cholinergic agonist pyridostigmine nine healthy women were challenged with both active drug and placebo at three time points in two consecutive menstrual cycles: a total of six neuroendocrine tests. A randomized, double-blind, counterbalanced design was used. Subjects were tested in the early follicular, mid-cycle, and luteal phases of the cycle. A cannula was inserted in a forearm vein after an overnight fast and baseline GH, estradiol, and progesterone samples were drawn. After 120 mg oral pyridostigmine or placebo tablets further blood samples for GH analysis were drawn at intervals over 3 h. When expressed as maximum change from baseline (delta GH) mean GH responses to pyridostigmine increased incrementally from early (8.4 +/- 2.7 micrograms/L) through mid (18 +/- 1.3 micrograms/L) to late (22.2 +/- 1.9 micrograms/L) cycle. This represents a significant effect of cycle phase on the GH response to pyridostigmine (P less than 0.001, as assessed by analysis of variance). Responses to placebo did not vary. Plasma estradiol values were significantly correlated with GH responsivity to active drug throughout the cycle (P less than 0.02). Multiple regression analysis also revealed a significant positive correlation between progesterone levels and GH response to pyridostigmine (P less than 0.02). Estrogens augment GH responses to other challenges but a priming effect of progesterone on GH responsivity has not previously been demonstrated. Various mechanisms are discussed including a possible sex steroid priming effect on acetylcholine neurotransmission.