In this paper I shall discuss the factors in the regulation of body temperature in humansthat have particular signifiance for old people, but to do this I must start from what isknown about the response to the thermal environment in old mice. This means that I shall touchonly lightly on certain factors about which much is known from human experience, but on theother hand, my vantage point in animal research may enable me to bring out some factors thathave not yet received attention.