AbstractThe amount of reducing equivalents from NADPH generated by glucose 6‐phosphate dehydrogenase activity (G6PD) used in mixed function oxidation (pathway I) or in reductive biosynthesis (pathway II) has been determined by cytochemical methods and microdensitometry in cells from the pars recta (PR) and distal convoluted tubule (DCT) of the kidney and from centrilobular (CL) and periportal (PP) hepatocytes from rats fed a normal or a vitamin D‐deficient diet. In the kidney, pathway I activity was similar to that of pathway II in PR, whereas in DCT pathway II was markedly predominant. Feeding a vitamin D‐deficient diet resulted in an increase in the total amount of reducing equivalents in PR and DCT. This increase was due to a rise in pathway I activity in the PR, whereas in the DCT the increase resulted from a stimulation of pathway II activity. Pathway I activity in PR was inversely correlated with plasma calcium, and was significantly decreased when calcium (1 mM) was added in vitro. In the liver the total amount of reducing equivalents generated by G6PD and both hydrogen pathways, was higher in CL than in PP hepatocytes. In CL cells, a vitamin D‐deficient diet induced a significant increase in both NADPH pathways. Furthermore, in these cells pathway I activity was inversely related to plasma calcium and was significantly lowered when 1 mMcalcium was addedin vitro. It is concluded that vitamin D status and calcium influence the production and utilization of cytosolic reducing equivalents both in kidney an