Biology of Reproduction, Vol 15, 158-162, Copyright © 1976 by Society for the Study of Reproduction

Diurnal Variations of Plasma Testosterone in Stallions

¹ Department of Veterinary Science, University of Kentucky, Lexington, Kentucky 40506

Plasma testosterone was measured in the peripheral blood of normal stallions by radioimmunoassay. In 24 stallions a single blood sample was taken between 9:30 and 10:30 a.m. and the mean testosterone concentration was 3.06 ± 1.27 ng/ml. In the other 8 stallions, blood was withdrawn every 4 h. In all 8 stallions there was evidence of a diurnal rhythm. The maximum mean blood plasma concentration of testosterone was observed in the sample collected at 8:00 a.m. and the minimum at 8:00 p.m. These differences in the blood plasma testosterone level are significant (P<0.10).

Note:
ACKNOWLEDGMENTS The testosterone antisera (#667) used in the radioimmunoassay was kindly provided by Dr. Gordon Niswender of Colorado State University, Fort Collins, Colorado, U.S.A. The investigation reported in this paper is in connection with a project of the Kentucky Agricultural Experiment Station, supported by a grant from The Grayson Foundation, Inc., and is published as paper No. 75-4-176 with the permission of the Director of the Station.