Weanling rats were fed for 6 months on a diet deficient in essential fatty acids: either fat-free, or with 28% (w/w) partially hydrogenated fish oil. Control rats were fed a diet with 28% (w/w) arachis oil for 6 months. 15-Hydroxyprostaglandin dehydrogenase activity was determined as initial rates of formation of H-labelled 15-keto-dihydro-prostaglandin E plus 15-keto-prostaglandin E in high speed supernatants of lung and kidney from each of the groups of rats. Dehydrogenase activity (expressed as either pmol/min per mg soluble protein, or as nmol/min per g tissue) was decreased 30-40% in the lungs of the two groups on diets deficient in essential fatty acids as compared to the control group. No difference was observed in dehydrogenase activity in the kidneys. The dehydrogenase may be of importance for the regulation of the level of endogenous prostaglandins and, thus, a decrease in activity could result in a slower turnover of prostaglandins.