Abstract—Ribosomes isolated from the brains of rats treated with morphinein vivowere less active in promoting the incorporation of [14C]leucine into protein than ribosomes isolated from untreated rats. This inhibitory phenomenon was studied in relation to dose of morphine, time after drug administration and the pharmacological responses of hypothermia and analgesia. The inhibition of [14C]leucine incorporation into brain proteinsin vitrowas transient after a single injection of morphine and dose‐dependent, and related to the hypothermic response, but not prevented by keeping the rats at an ambient temperature which prevented hypothermia. The incorporation of [14C]leucine into protein by liver ribosomes was also inhibited in preparations from morphine treated r