The capacity of human skeletal muscle and kidney homogenates to synthetize prostaglandins (PGs) from exogenous precursor was investigated. Low-speed supernatants of muscle as well as renal medullary and cortical homogenates were incubated with 14C-labelled arachidonic acid (14C-AA) prepared as a sodium salt. 14C-PGs in the incubates were extracted, separated with thin-layer chromatography (TLC) and quantified by radioscanning. In the skeletal muscle incubates 14C-AA was converted into 14C-PGs with a time-dependent yield, most effectively after 10--15 min incubation. Well-defined radiopeaks parallel to unlabelled standards of PGD2, PGE2, PGF2 alpha and 6-keto-PGF1 alpha were obtained in the chromatograms. PGE2 was the main PG formed, constituting over 50% of 14C-activity, whereas 6-keto-PGF1 alpha, PGD2 and PGF2 alpha were found in considerably lower proportions. In the renal medullary incubates, PGE2 likewise accounted for the largest part of 14C-PGs formed, but significant relative amounts of PGF2 alpha and PGD2 were also found. A minor peak, corresponding to 6-keto-PGF1 alpha and thus indicating formation of PGI2, was also obtained. In contrast to the medulla, no 14C-PGs could be found in the renal cortical incubates. The results demonstrate the existence of a considerable tissue specificity in the quantitative and qualitative expression of PG biosynthesis in man.