19-nor-deoxycorticosterone (19-nor-DOC) is a potent salt retaining and hypertensinogenic mineralocorticoid that is excreted in the urine. While the precursor of 19-nor-DOC, 19-oxo-DOC, is produced by the adrenal cortex, conversion to 19-nor-DOC does not occur in the adrenal gland. We have examined the hypothesis that 19-nor-DOC is synthesized from precursors in the kidney. 19-oxo-DOC was added to the perfusate of isolated rat kidney preparations (n = 5) at a concentration of 10 microM. During 1 h of perfusion following addition of 19-oxo-DOC, 71 +/- 6% of the precursor was converted to 19-oic-DOC, an immediate precursor of 19-nor-DOC, and 8.3 +/- 1.8% was converted to 19-nor-DOC. This represents the first definitive evidence that 19-nor-DOC is produced in the kidney from adrenal precursors.