Abstract 10.1002/zoo.1430090405.abs Testosterone concentrations in serum samples collected weekly over a 5-year period from a young adult male Asian elephant (Elephas maximus) and a young adult male African forest elephant (Loxodonta africana cyclotis) were measured by radioimmunoassay. Testosterone profiles during this maturational period were compared between the two species and related to the occurrence of musth, a recurring physiological and behavioral condition exhibited by most mature Asian, and some African, bull elephants. Musth is characterized by secretion from the bull's temporal glands, dribbling urine, and increased aggression. Serum testosterone concentrations in the Asian bull were elevated substantially between April and September each year, coincident with the presence of temporal gland secretion, urine dribbling, and aggressive behavior. Testosterone levels from April through September averaged (± SEM) 41.2 ± 2.8 ng/ml, compared to 7.6 ± 1.0 ng/ml during the rest of the year. In contrast, the testosterone profile of the African bull showed greater variation and lower levels overall, the only pattern being a tendency for levels to be lowest from November to February (avg. 6.8 ± 1.5 vs. 10.3 ± 0.8 ng/ml during the rest of the year). Temporal gland secretion and other signs of musth were first observed in this bull in 1988, at age 17. While his testosterone profile did not show a pattern comparable to that in the Asian bull, average testosterone values were significantly greater in 1988 compared to previous years. The Asian bull showed sexual attention to preovulatory (estrous) cows whether in musth or not, and exposure to estrous cows did not appear to alter the highly consistent, annual pattern of musth as evidenced in temporal gland flow.