1993 Volume 39 Issue 5 Pages j17-j21
Summary. Blood samples were taken from 22 stallions at hourly intervals for a 24-h period in all seasons, and plasma testosterone (T) and dihydrotestosterone (DHT) were measured by RIA. The stallions aged 11-17 (11-17 year old group) showed a biphasic diurnal variation in T in all seasons. The first peak appeared in the daytime and the second peak, at night. The time of the first peak was correlated with sunrise (r=0.69; P<0.05). DHT remained low and no peaks could be detected. For stallions aged 20-21 (20-21 year old group) no distinct biphasic shifts in T could be detected. The 24-h mean concentrations of T and DHT in the 11-17 year old group were highest in the summer and lowest in the fall (T, P<0.01; DHT, P<0.05), but that of T in the 20-21 year old group was highest in the spring and lowest in the fall (P<0.05). When T 24-h mean concentrations of the groups in the same season were compared, significant differences were noted for the summer (P<0.05), but not in any other season. It is concluded that in stallions, diurnal variation and 24-h mean concentration of T are affected by age and season.
