OBJECTIVE Critical illness has been associated with decreased LH secretion and lowering of testosterone. Dopamine is often used for vasoactive support in these patients. We therefore aimed to investigate LH secretion during severe illness and particularly the effect exerted by dopamine on LH in such patients. METHODS In a randomized, controlled study of critically ill adult male polytrauma patients (n = 9), we evaluated the effect of prolonged (83-296 hours) dopamine infusion (5 micrograms/kg/min i.v.) on the dynamics of LH secretion and on serum testosterone concentrations. The effect of brief (15-21 hours) dopamine administration was documented in an additional randomized, controlled, cross-over study involving 6 patients. METHODS Serum LH concentrations were measured by IRMA. The LH profiles, obtained by blood sampling every 20 minutes for 9 hours during two consecutive nights, were examined by deconvolution analysis. Serum testosterone concentrations were measured by RIA once per study night. RESULTS We found that before dopamine initiation and within 24 hours of dopamine withdrawal, the mean serum LH concentrations, the LH secretory amplitude, the amount of LH secreted per burst, the mean LH secretion rate and the number of LH pulses were higher than during dopamine infusion, being increased by a median of 161% (P = 0.006), 98% (P = 0.03), 106% (P = 0.03), 164% (P = 0.01) and 25% (P = 0.008) respectively. However, without dopamine administration the amplitude and mass of the LH secretory bursts still appeared to be low, whereas the pulse frequency remained elevated. After dopamine withdrawal, LH secretion increased significantly within 3 hours. Serum testosterone levels were very low and dopamine infusion appeared not to affect them within 24 hours. CONCLUSIONS We documented decreased LH secretory pulse amplitude and mass with increased pulse frequency, as well as very low serum testosterone concentrations in critically ill men. Dopamine infusion further suppressed LH release by decreasing secretory burst amplitude, mass and frequency, possibly through an inhibitory action at both the pituitary and the hypothalamic level.