1. Circadian rhythms of blood pressure, heart rate, and core body temperature have been shown to be strongly entrained by food intake such that feeding restricted to 4 hours during daytime (from 10 AM to 2 PM) was associated with a prompt shift of peak values in all three variables from the dark period to the daytime. Surprisingly however, daytime restricted feeding had little effect on voluntary locomotor activity as assessed by running wheel activity, the most commonly used method to determine circadian rhythmic patterns. Peaks of running wheel activity during 6 days of daytime restricted feeding continued to occur with onset of the dark period with a small second activity around the time of feeding. Peak night time activity was seen in the first half of the dark period and this activity fell slightly with the duration of daytime feeding. Nevertheless, the dissociation in the entrainment of cardiac and metabolic variables on the one hand and voluntary running activity on the other suggests a complex interaction between clock gene outputs and behavioral adjustments. 2. Determination of food intake during daytime restricted feeding showed that the 4 hour feeding regimen used in our studies is associated with a sub-caloric energy supply. Body weight over a 6 day period fell from 27.4 to 25 g and food intake was about 20% less than during ad libitum feeding. This energy deficit was even more pronounced in diabetic Akita mice in which body weight fell from 23.4 to 19.4 over 6 days, and this was associated with dramatic reductions of core body temperatures during the dark phase. Stable body weights and higher food intakes were found when the food restriction was extended from 4 to 6 hours (from 9 AM to 3 PM). Extended feeding during the daytime markedly reduced the fall in body temperature in the night time observed during restricted feeding in wild type and diabetic animals suggesting that in addition to food-dependent entrainment of daily rhythms caloric energy supply determines the magnitude of the rhythmic perturbations.