Adult Hymenolepis diminuta mitochondria catalyze a transhydrogenation reaction between NADPH and NAD and between NADH and NAD. The NADPH-->NAD reaction is catalyzed by an inner membrane-associated pyridine nucleotide transhydrogenase, whereas the NADH-->NAD reaction is ostensibly catalyzed by another system(s). The source(s) of NADH-->NAD activity was evaluated by assessments of its intramitochondrial distribution and thermal lability and by comparisons with the distribution/thermal lability of NADH dehydrogenase, lipoamide dehydrogenase, and NADPH-->NAD transhydrogenase. The occurrence of NADH and lipoamide dehydrogenase components was readily demonstrable. Like NADPH-->NAD transhydrogenase, NADH dehydrogenase was essentially membrane bound. Lipoamide dehydrogenase and NADH-->NAD activities were, at different levels, in the membrane and soluble fractions. Based on thermal profiles, NADH and lipoamide dehydrogenase differed from each other and from NADPH-->NAD transhydrogenase. Although the NADH-->NAD profile closely paralleled that for lipoamide dehydrogenase, it also was similar to the NADH dehydrogenase profile. Collectively, these data are consistent with the supposition that the H. diminuta mitochondrial NADH-->NAD transhydrogenation reaction is catalyzed by lipoamide dehydrogenase and possibly by NADH dehydrogenase rather than by an independent transhydrogenase system.