This study was designed to develop a method of rapidly evaluating dietary proteins for channel catfish (Ictalurus punctatus) and to gain basic knowledge of its nitrogen metabolism with respect to dietary protein quantity and protein quality. Experiments l, 2 and 3 were short-term (7-week) growth trials with fingerling catfish in aquaria under controlled laboratory conditions. Parameters measured each week included instantaneous daily gain of the fish (IDG) and the activities of selected enzymes - glutamate dehydrogenase (GDH), aspartate aminotransferase (GOT), alanine aminotransferase (GPT) and alkaline phosphatase (AP). Fish were fed purified diets differing in protein quantity and protein quality in experiments 1 and 2, respectively. GDH, GOT, GPT and AP activity and IDG increased with increased dietary protein quantity. The consistency of the relationship between enzyme activity and IDG decreased as follows: GOT, GDH, GPT and AP. GOT activity and IDG increased, and GDH activity decreased, with increased dietary protein quality. IDG was positively correlated to GOT activity and negatively correlated to GDH activity. GPT activity was unrelated to either dietary protein quality or IDG. GDH and GOT activity responded to changes in dietary proteins within 3 weeks in experiments 1 and 2. The ratio of GOT activity to GDH activity was positively correlated to dietary protein quality. The growth and enzyme activity data from experiments 1 and 2 were used to develop indices of growth based on enzyme activity. The growth indices were tested as a method of rapidly evaluating dietary proteins in experiment 3 where fish were fed natural ingredient diets differing in protein source. The proteins included fish meal and four types of seafood processing wastes. GDH, GOT and GPT activity, the GOT/GDH ratio, IDG and the growth indices were similar among treatments in catfish fed different protein sources at 10 percent of the diet in experiment 3. The results of experiment 3 could not be used to adequately assess the growth indices and the GOT/GDH ratio but did indicate the feasibility of using seafood processing wastes as fish meal substitutes at 10 percent of the diet.