Intravenous administration of a single dose (20 micrograms) of recombinant interleukin-1-beta to virgin, lactating and litter-removed rats rapidly decreased intestinal lipid absorption in all groups. In vivo, oxidation of [14C]triolein to 14CO2 was also significantly decreased by interleukin-1. In addition, the cytokine decreased [14C]lipid accumulation in the mammary gland of lactating rats and in the adipose tissue of virgin and litter-removed rats. The decrease in lipid uptake in the interleukin-treated rats was accompanied by hypertriglyceridaemia; however, there was no significant decrease in tissue lipoprotein lipase activity, except in heart from lactating rats. In contrast, interleukin-1 administration had no effect on lipogenesis in liver, white or brown adipose tissue of virgin rats fed on glucose. These results suggest that interleukin-1 profoundly affects lipid metabolism by delaying intestinal absorption and decreasing tissue uptake.