SummaryThe effects of diet restriction, thyroidectomy, adrenalectomy, and ambient temperature (28 and 34°) on protein turnover rate (K) were studied in young (47–49 days) and old (572–605 days) male rats. The data indicate that thyroidectomy decreased K under all treatments. Adrenalectomized rats supplemented with salt and 300 μg corticosterone every second day had a similar K and growth rate as the control rats. Exposure to heat and restriction of diet decreased K significantly only in the young rats. The effect of heat is suggested to be associated with the voluntary reduced feed intake as a result of hyperthermia and depressed appetite.