Determinations of spontaneous activity in rats kept in recording cages disclosed no effects from thyroid or parathyroid extirpation or thyroid feeding. Adrenal extirpation often induced adynamia of from 2 weeks to permanent duration. Ergographic studies showed reduction of work capacity of individual muscles to 1/16 normal. Glycerine extracts of adrenal gland (whole or cortex, only) by mouth or subcu-taneously had no beneficial influence on voluntary activity. Females were found to be spontaneously more active than males, hence the ovarian hormone is presumably more potent than that of the testis. Castration and spaying resulted in marked decrease of spontaneous activity and of work capacity of individual muscles. Hysterectomy had no demonstrable influence. As shown by Wang, activity could be restored approximately to normal by ovarian grafts. Oral administration of glycerine extract of whole ovary, subcutaneous injection of saline extract of corpus luteum, saline prostatic extract and numerous sorts of testicular extracts, as well as testis feeding and vasectomy, had no invigorating effect. Testis grafts gave almost but not quite completely negative results. No constant correlation between changes of basal metabolism and voluntary activity was discovered.