We examined meal patterns after isocaloric duodenal infusions of fat, carbohydrate (CHO), and protein by measuring meal size, intermeal interval (IMI) and total food intake (TFI). Wistar rats were adapted to normal feeding 6 h/day, with continuous computer monitoring of feeding patterns. One of five solutions (10 ml of 1 kcal/ml at 0.45 ml/min; 0, 20, 50, 80, or 100% of energy from fat) or saline (control) was infused 10 min after initiation of eating. Separate rats received casein or casein hydrolysate at 18.5 or 37% energy. Equivalent energy loads varying in fat, CHO, and protein content compared with saline resulted in similar reductions in first meal intakes. The second meal did not differ among fat and CHO treatments including saline; however, infusion with a protein-containing solution increased the size of meal 2. The IMI was doubled by protein infusion independently of dose or source but extended dose dependently by fat. TFI was lower after high fat and higher after protein than after saline infusion. The results indicate that the concentrations of fat, CHO, and protein differentially affect the qualitative and quantitative aspects of feeding in rats.