A master circadian pacemaker located in the suprachiasmatic nuclei (SCN) controls circadian rhythms in mammals. This pacemaker receives monosynaptic input from the eyes, which allows it to be entrained by environmental cycles of light and darkness. Another circadian pacemaker, whose neural substrate has not yet been identified, can control behavior in the absence of the SCN and can be entrained by environmental cycles of food availability. Because this second pacemaker is responsive to food but not to light, it has been called the food-entrainable oscillator. By contrast, the SCN pacemaker has been referred to as the light-entrainable oscillator, even though there is substantial evidence that it is responsive to food as well as to light. Interestingly, studies still ongoing in my laboratory seem to suggest that the food-entrainable oscillator is responsive not only to food but also to ambient temperature. Mice have been kept in individual cages with access to running wheels. Their rhythms of locomotor activity have been consistently shown to free-run in constant darkness and to entrain to environmental cycles of light and darkness, of food availability, and of ambient temperature. When two different environmental cycles (for example, light and environmental temperature) were presented simultaneously with different cycle lengths (for example, 23 h and 24 h), the behavior of the mice showed a dissociation into two components that seemed to reflect the separate entrainment of two pacemakers, presumably the light-entrainable oscillator and the food-entrainable oscillator. The possibility that one of the behavioral components reflected merely masking has been partially but not yet fully eliminated. Because the SCN pacemaker is preferentially entrained by light, the dissociated pattern of behavior implies that the food-entrainable oscillator can be entrained not only by a cycle of food availability but also by a cycle of ambient temperature.