SummaryobjectiveThe aim was to investigate whether a pulsatile discharge of LH from the pituitary Is necessary to achieve the circadian secretion of testosterone.designThe daily rhythm of the androgen has been studied in patients with idiopathic hypogonadotrophic hypogonadism (IHH) both in the absence of therapy and during pulsatile administration of gonadotrophln releasing hormone (GnRH).patientsSix patients with IHH and ten normal subjects were analysed. Blood sampling was performed at 2‐hourly intervals, for 24 hours. The IHH patients then received synthetic GnRH i.v. at the rate of one pulse every 2 hours (10μg/pulse). On day 11 of treatment, blood samples were taken for the rhythm analysis every 2 hours, for 24 hours.measurementsPlasma testosterone and LH were measured in the individual samples by radioimmunoassay. Evaluation of the rhythm was performed by cosinoranalysis.resultsA significant circadian rhythm of plasma testosterone was statistically validated In the normal subjects, whereas no rhythm was detected in the IHH patientsin the absence of therapy. On day 11 of GnRH pulsatile administration the IHH patients showed normal testosterone levels and a statistically indistinguishable from the corresponding values in the normal subjects. Plasma LH did not show statistically significant circadian variations, either In the control group or in the IHH patients before or during therapy.conclusionsWe conclude that a physiological circadian rhythm of plasma testosterone can be obtained, In IHH men, by treatment with GnRH. Since the pulsatile administration of exogenous GnRH at constant dosesinduced a circadian rhythm in testosterone and no dally variations in LH were evident, we suggest that, although a pulsatile secretion of LH is probably necessary for the synchronization of the circadian rhythm with acrophase inthe morning, the testosterone variations might be theresults of a local testicular modulation of LH acti