The effect of lowered body temperature on lifespan and immune and nonimmune processes in invertebrates and vertebrates are reviewed. Decreasing the environmental temperature tends to increase lifespan of invertebrates and poikilothermic vertebrates. Thermolabile homeotherms appear to live longer than comparably-sized species which do not undergo periodic, natural lowering of their body temperature. Even a mild lowering of temperature suppresses both humoral and cellular immune processes in poikilotherms. On the other hand, metabolic and biochemical responses to decreased body temperature in poikilotherms are quite varied. With mild or moderate hypothermia (30–35°C), indications are that for most homeotherms, there would not be a major inhibition of nonimmune biochemical or metabolic processes. Initial studies aimed at inducing mild, chronic or intermittent hypothermia in mice with chloropromazine and marihuana derivatives are described.