Reproductive physiology of the stallion. IV. Seasonal changes in the testosterone concentration of peripheral plasma.

The normal equine breeding season begins in the spring and extends through mid-summer. It is characterized by marked increases in sperm output and seminal volume, as well as behavioural changes manifested in decreased reaction time and the number of mounts per ejaculation (Pickett & Voss, 1972). The production of androgenic steroids by the equine testis has been investigated both in vivo (Lindner, 1961) and in vitro (Savard & Goldzieher, 1960; Bedrak & Samuels, 1969; Oh & Tamaoki, 1970) but the testosterone level in peripheral plasma has not been reported (Lindner, 1961) and seasonal influences upon testicular function have received little attention. The object of this study was to establish the influence of season on the concentration of testosterone in the peripheral plasma of the stallion. Two ejaculates were collected from five mature Quarter Horse stallions each Tuesday. Blood was withdrawn from the jugular vein of each animal at approxi¬ mately 10.00 hours on the Wednesday closest to the middle of the month and