Plasma concentrations of testosterone in the male dog and plasma testosterone profile following single intramuscular injection of HCG.

The spontaneous variation in plasma testosterone was studied in 4 dogs in a 24 h period. Blood samples were taken with 1½ h interval. A variation of 26–62 was found in the plasma testosterone concentration and the values ranged from 2.7–15.6; 0.7–10.4; 4.2–17.3; 8.7–23 nmol/l. No effect of intramuscular injection of 150 i.u. HCG could be seen on the plasma testosterone levels in samples taken 5, 10, 20 or 30 min post injection. Thereafter plasma testosterone levels increased to reach levels equal to peak concentration in the control period 1½ h post injection. The variation in the plasma testosterone concentration 1½, 3 and 4½ h post HCG injection was reduced to 8.95 ± 2.8 % (mean ± s). Plasma testosterone in a sample taken 3 h after HCG injection might thus be indicative of a Leydig cell dysfunction in the dog. An additional increase (P < 0.001) in plasma testosterone levels was observed 21–30 h post injection. Thereafter levels of testosterone fell to pre-stimulation values.