Abstract 1. 1. A spectrophotometric assay using p-iodo-nitro-triphenyl tetrazolium chloride as the indicator was developed for splenic tissue in the goldfish Carassius auratus. 2. 2. This technique permits a rapid test for the activities of the enzymes lactic dehydrogenase, malic dehydrogenase, and α-glycerophosphate dehydrogenase which are indicative of the relative activities of various metabolic pathways.