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=.\2 to 1\m=.\4 ng/ml) ; as they began to come into musth and the temporal glands started to enlarge, the testosterone levels rose (4\m=.\3 to 13\m=.\7ng/ml), and when the animals were in full musth, with discharging temporal glands and an aggressive temperament, the levels were extremely high (29\m=.\6 to 65\m=.\4 ng/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.

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