Five purified diets were utilized to investigate the effect of varying protein and energy intake on postprandial changes in systemic serum amino acid levels of channel catfish ( Ictalurus punctatus). The diets contained 25, 30 or 35% crude protein at one of three different energy levels. Systemic serum free amino acids and glucose were determined at 0, 2, 4, 8, 12 and 24 h after feeding each diet. In general, for each diet tested, most of the systemic serum free amino acid levels increased within 2 h after feeding, remained elevated for up to 12 h, and returned to fasting levels within 24 h after feeding. Varying the protein to energy ratios in the diets did not profoundly affect postprandial systemic serum free amino acid patterns. There was a significant positive correlation between the levels of systemic serum free essential amino acids and dietary levels. No such correlation was observed for non-essential amino acids. Serine and alanine were the most abundant free amino acids in catfish serum. Glutamic acid, glycine and proline remained relatively low throughout all treatments. A reciprocal relationship was observed between levels of glucose and total free amino acids in the serum.