Anorexia, nausea, and vomiting are common symptoms of uremic intoxication. Fractions in the middle molecule weight range, isolated from normal urine and uremic plasma ultrafiltrate, inhibit ingestive behavior in the rat. To investigate their site of action and specificity, male rats were injected intraperitoneally, intravenously, or intracerebroventricularly with concentrated fractions of uremic plasma ultrafiltrate or normal urine (molecular weight range: 1.0 to 5.0 kD) and tested for ingestive and sexual behavior. An intraperitoneal injection of 0.5 ml of urine fraction (10:1) or 2.0 ml of uremic plasma ultrafiltrate fraction (25:1) inhibited carbohydrate intake by 76.3 and 45.9%, respectively, but an intravenous injection had no effect. However, intravenous injection of higher doses inhibited carbohydrate ingestion. An intracerebroventricular injection of 5 or 10 microl of urine (20:1) middle molecule fraction inhibited carbohydrate intake by 13.4 and 41.6%, respectively. An injection of 5 or 10 microl of uremic plasma ultrafiltrate (125:1) middle molecule fraction inhibited carbohydrate intake by 22.6 and 49.5%, respectively. Injections of the corresponding fraction from normal plasma ultrafiltrate had no effect. Injection of urine or uremic plasma ultrafiltrate middle molecule fractions did not affect the display of sexual behavior. These results suggest that middle molecule fractions from uremic plasma ultrafiltrate or normal urine act in the splanchnic region and/or brain to inhibit food intake and that the effect is specific for ingestive behavior.