Summary. Testosterone was measured in the peripheral blood plasma of eleven male Asiatic elephants, using a competitive protein-binding assay. When the animals showed no signs of musth, the testosterone levels were low (<0\m=.\2to 1\m=.\4ng/ml) ; as they began to come into musth and the temporal glands started to enlarge, the testosterone levels rose (4\m=.\3to 13\m=.\7ng/ml), and when the animals were in full musth, with discharging temporal glands and an aggressive temperament, the levels were extreme- ly high (29\m=.\6to 65\m=.\4ng/ml). Musth may therefore be comparable to the rutting behaviour of some seasonally breeding mammals, although, in the elephant, there is some indication that it may be induced by sexual activity.