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The aim of the investigation was to study the effects of ACTH 1-17 on plasma testosterone, plasma aldosterone as well as on both plasma and urinary electrolytes (K, Na, Mg and Ca) in healthy young adult males with regard to the time (clock hours) at which this polypeptide was injected. Eight healthy adults (males from 28 to 30 years) volunteered for the study. The were synchronized with a diurnal activity from 0700 to midnight and a nocturnal rest. Each week, during 6 consecutive weeks (January 19 to February 25, 1980) a 3-day test was performed on Saturday, Sunday and Monday. On Sundays 3 control-tests and the 3 ACTH-tests were programmed during which either saline or 100 microgram ACTH 1-17 were injected i.m. at respectively 0700, 1400 and 2100. During each 3 day-test period (72 h) the urinary excretion of K, Na, Mg and Ca was determined every 4 h at fixed clock hours. In addition, on Sundays, venous blood was sampled prior to control or ACTH injections at respectively 0700, 1400 and 2100 and 20, 40, 60, 90, 120, 150 and 180 min thereafter. Plasma testosterone, aldosterone (radioimmunoassays) K, Na (flame photometry), Mg and Ca (photocolorimetric methods) were determined in the collected samples. Both conventional and cosinor methods were used for statistical analyses. The injection of ACTH at 0700 was followed by a clear and statistically significant rise of plasma testosterone. No change with regard to control occurred when ACTH was injected at either 1400 or at 2100. A statistically significant rise of plasma aldosterone was observed after each of the ACTH injections. However, the highest plasma aldosterone level was reached when ACTH was administered at 1400 and the lowest level at 2100. ACTH-induced changes in plasma electrolytes were either nil (for Na and Ca) or small (for K and Mg). A more or less important increase of urinary K occurred after the ACTH injection at each of the 3 considered times. The highest values of excreted K occurred after the injection of ACTH at 0700, without shift of the acrophase. In contrast, injections of ACTH at 1400 and 2100 induced a dramatic alteration of the K rhythms. ACTH induced an important fall in the Na urinary excretion. This fall was the greatest when ACTH was injected at 1400. Na rhythm alterations also occurred, particularly after ACTH injections at 2100. However, this effect was less pronounced after ACTH injection at 0700 than at other considered time points. The urinary amount of excreted Ca did not seem to be affected by ACTH. Rhythm alterations occurred after ACTH injections at 1400 and 2100. Peaks of plasma testosterone, plasma aldosterone as well as plasma cortisol (reported in a previous paper) resulting from ACTH stimulation coincided in time with the acrophase of the physiological circadian rhythm in plasma levels of these hormones...