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Summary. A result reported earlier that oestradiol sensitizes the nervous system of the male rat so that it responds to testosterone with increased motor activity could not be repeated. The male and female rat respond differently, the male only to oestradiol and the female to both oestradiol and testosterone. A few rats sometimes respond to testosterone. Oestradiol benzoate pellets implanted beneath the skin of the gonadectomized rat cause both males and females to take large amounts of exercise in an activity drum. Testosterone propionate causes the females to take much exercise but has a small effect upon the males. Asdell, Doornenbal & Sperling (1962) have suggested that testosterone propionate is effective in males if the implant follows one of oestradiol benzoate. Therefore it was postulated that oestrogen sensitizes the nervous system to androgen. We have been unable to repeat this work and believe that the previous result was obtained because the testosterone implant was made before the direct effect of the oestrogen had disappeared. Eight male rats were castrated when about 45 days old. A week later, each was implanted subcutaneously with one pellet containing 1 mg of oestradiol benzoate. All responded with increased activity and in all but one a daily average of over 10,000 rotations of the activity drum, about 7 miles, was reached. At the end of 8 weeks, when activity had decreased to normal levels, each rat was implanted with one pellet containing 1 mg of testosterone propion¬ ate. During the following 8 weeks, no increased activity was observed in any of the rats. They were then implanted with 1-mg oestradiol pellets and all displayed increased activity but not to the degree reached in the earlier oestrogen period. After 8 weeks, another testosterone implantation was made. Activity increased slightly in two rats only. In an experiment designed to test the effect of testosterone level, eight male rats were castrated when about 45 days old. Two were implanted with single 1-mg testosterone propionate pellets. One showed a marked increase in activity; the other had a small response. With two 1-mg pellets, one rat increased markedly and the other showed a slight response. With a single 10-mg pellet, one rat responded markedly; the second with a little increased activity. With two 10-mg pellets, one rat increased markedly and the second to a small degree. After 8 weeks, 1-mg testosterone propionate pellets were

SEX STEROID HORMONES AND VOLUNTARY EXERCISE IN RATS. A CORRECTION