The biochemical response of rat splenic D-T diaphorase and the histochemical distribution of the enzyme NAD(P)H-NBT reductase to the action of the polycyclic hydrocarbons benz(a)pyrene, 3-methylcholanthrene, 7,12-dimethylbenz(a)anthracene and benz(a)anthracene have been studied. The four polycyclic hydrocarbons tested in this work induced the activity of both enzymes. The stimulation of the D-T diaphorase by benz(a)pyrene is dose dependent and it is partially inhibited by dicumarol. Microsomal and mitochondrial NAD(P)H dehydrogenases are not induced by any of these compounds. The study of the histochemical distribution of the NAD(P)H-NBT reductase shows also a marked increase in the staining of the enzyme which follow a specific pattern, the cells showing the highest activity are the lymphocytes located around the marginal sinus of the white pulp and around follicular arterioles, plus red pulp lymphocytes and myeloblastic cells. The cells in the germinal center show from null to very weak activity. A correlation between the biochemical induction of the soluble D-T diaphorase of the histochemical increase of the NAD(P)H-NBT reductase is attempted.