The properties of the four most commonly used tetrazolium salts, neotetrazolium, nitro blue tetrazolium (nitro-BT), tetranitro-BT, and 2-(2-benzothiazolyl-3-(4-phthalhydrazidyl)-5-styryl-te trazolium (BPST), have been compared for their effects on the localization of nicotinamide adenine dinucleotide phosphate (NADP)-dependent dehydrogenases under optimal incubation conditions in cryostat sections of rat liver. Glucose-6-phosphate dehydrogenase has been selected as an example of these dehydrogenases. It was found that the use of nitro-BT and tetranitro-BT, unlike neotetrazolium and BPST, in combination with an exogeneous electron carrier and azide, resulted in localization patterns in agreement with the sites of activity as determined by microchemical techniques. In the absence of an intermediate carrier the localization was very similar to that of NADPH cytochrome c (P450) reductase as demonstrated immunocytochemically. BPST did not properly localize dehydrogenase activity, most probably because of the redistribution of formazan, due to its lack of firm substantivity. Neotetrazolium reduction in nitrogen gave the localization pattern, both in the presence and absence of carrier, of the reductase, suggesting that the transfer of reducing equivalents from the exogenous electron carrier to neotetrazolium proceeds via cellular electron transport systems. The reduction of nitro-BT and tetranitro-BT via intermediate carriers was oxygen sensitive in parenchymal cells, but not in the non-parenchymal liver cells. This oxygen sensitivity could be blocked by azide. With neotetrazolium, oxygen inhibited both carrier-mediated and carrier-independent reactions, effects that were not reversed with azide. Possible mechanisms of action between oxygen, reduced carriers, and tetrazolium salts are discussed.