Dietary restriction (DR, also referred to as calorie restriction, energy restriction, and food restriction) retards senescence and increases longevity in mammals. DR also lowers mean body temperature (Tb), and thus mean Tb might be useful as a covariate of DR-induced life extension. Indeed, lower Tb could itself underlie some of the beneficial life-extension effects that occur during DR. To assess the relationship between lower Tb during DR and life extension, we asked whether significant strain variation exists in the Tb response of mice being fed 60% ad libitum (AL). Individually-housed, female mice from 28 strains, representing a genealogically diverse sample of the classical inbred strains, were directly compared. The mean Tbs in response to DR exhibited highly significant strain variation, ranging from 1.5°C below normal to a phenomenal 5°C below normal. This variation was not explained by differences in loss of thermoregulation, AL adiposity, sensitivity to a nonadaptive hypothermia, motor activity, thermal arousal, absolute food intake, or efficacy of nutrient extraction. The variation in strain mean Tb was also present in the absence of torpor. This strain variation could be used to critically test whether lower Tb is a covariate of life extension during DR. © 2003 Elsevier Science Ireland Ltd. All rights reserved.