The effect of a long-term 50% food restriction on plasma thyroid hormone levels was studied in adult male rats, which received their reduced daily amount of food either as one meal or 5 different meals. Compared to controls, a long-term food restriction reduced total 24 h-mean plasma T4, T3 and TSH concentrations. The total T4 level was more reduced in rats fed their reduced ration in one meal than in rats given food in several meals. This resulted in a higher T3/T4 ratio in the former group of rats than in the other groups (free access to food; food in several meals). In the control group and in the food-restricted group receiving their food in one meal, significant diurnal variations in total T4, T3 and TSH levels were seen. The T3 rhythm was abolished in food-restricted rats receiving their food spread over several meals, but T4 and TSH rhythms were not. We conclude that total peripheral thyroid hormone levels infood-restricted rats are influenced, not only by the amount of food but also by the feeding pattern.