The aims of the present study were to examine the variability of testosterone secretion in the Virginia Opossum over a 24 h period and to develop a testosterone stimulation test that would provide an index of the prevailing testosterone biosynthetic capacity of the testes; the latter was used to clinically evaluate the efficacy of a gonadotrophin-releasing hormone agonist contraceptive. Sexually-mature captive opossums (n = 12) located in Africam Safari (Mexico) sampled every 12 h over 24 h consistently showed basal (<0.21 ng mL(-1)) blood testosterone concentrations. Intra-muscular injection of buserelin (2 microg mL(-1)) and human chorionic gonadotrophin (hCG; 1000 IU) resulted in an increase (P < 0.05) of plasma testosterone concentrations with maximal concentrations (3.9 ng mL(-1) and 5.8 ng mL(-1) respectively) occurring 120 min after injection. Plasma testosterone declined relatively rapidly to basal concentrations after 240 min with hCG but remained elevated after the same period of time with buserelin. Male opossums treated with (n = 6) and without (n = 6) a controlled-release deslorelin implant (Suprelorin; 4.7 mg deslorelin) were evaluated over a 10-week period for changes in testosterone secretion (hCG stimulation test) and sperm production (spermatorrhea). At the end of this period, the animals were hemi-castrated and their relative testicular quantitative histology compared. Testosterone concentration decreased over the course of the study in both treated and control animals (P < 0.0001) but there was no apparent effect of deslorelin on testosterone secretion, testicular histology (relative proportions of testicular cell types and seminiferous tubule diameter), or sperm production (presence of sperm in the cauda epididymis or urine).