Effects of Mn2+ and Mg2+ as well as of o, p'-dichlorodiphenyl dichlorethane (o, p'-DDD) on NADP-isocitrate dehydrogenase (ICD) in adrenal gland cortex were studied in vivo. Maximal enzymatic activity in cytosol of adrenal gland cortex was found at 1.0 mM MnCl2 and 10.0 mM MgCl2 concentrations, pH7.4; activity of ICD was distinctly lower in the gland mitochondria than in cytosol. Km values of ICD from cytosol of human adrenal gland cortex were about 6.63 microM with NADP and 3.91 microM with DL-isocitrate, respectively. Feeding of dogs with o, p -DDD, which inhibits steroidogenesis, led to an increase in the ICD activity in homogenate of adrenal gland cortex and did not affect its activity in cytosol and mitochondria. It is unlikely that NADP-ICD serves as a donor of NADPH in reactions of steroid hydroxylation.