Abstract
Testosterone and 17.alpha.-hydroxyprogesterone were measured in the peripheral plasma of 6 young post-pubertal bulls of 1 yr of age, by separate radioimmunoassays. Samples were collected every hour for 25 h at the beginning of each of 4 seasons. On a separate occasion blood samples were collected from 1 bull every 10 min for 2 h. Testosterone and 17.alpha.-hydroxyprogesterone appeared to be secreted episodically. These 2 steroid peaks showed good correlation. Each 24 h period showed its own characteristic pattern of pulsatile changes. Episodic secretion seems to be progressive rather than rapid and short lived. No real circadian rhythm was observed but at about 10.00 a.m. a trough in these steroid secretions occurred. This was followed by an increase in peripheral plasma concentration. These troughs occurred at all seasons after the morning feed and while semen was being collected in the performance test station.