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1. Sodium transport across isolated frog skin, as measured by the short-circuit current, was decreased by acetylsalicylic acid, mefenamic acid, paracetamol and phenylbutazone. Indomethacin (6 X 10(-6) M) had a biphasic effect on the short-circuit current: a transient increase followed by a sustained decrease. 2. The release of prostaglandin-like material from the skin was reduced by acetylsalicylic acid and indomethacin. Paracetamol caused a significant reduction in the short-circuit current response of the skin to low doses of arachidonic acid, but the response to the highest dose tested was not significantly altered. 3. Indomethacin (6 X 10(-6) M) increased the sensitivity of the skin to applied prostaglandin E1. The other prostaglandin synthetase inhibitors did not have this effect. Indomethacin (6 X 10(-6) M) also enhanced the effect of antidiuretic hormone on the short-circuit current. 4. Indomethacin (30 X 10(-6) M) increased the short-circuit current and diminished the response to applied prostaglandin E1. 5. In sulphate Ringer, theophylline increased the short-circuit current and diminished the response to prostaglandin E1. 6. Prostaglandin E1 increased the levels of cyclic AMP in frog skin and these increases preceded the increases in short-circuit current. There was a seasonal variation in the level of cyclic AMP in the skin: the levels in winter exceeded those in summer. There was also a seasonal variation in the cyclic AMP response to prostaglandin E1: the winter response was greater than that in summer. 7. Indomethacin (6 X 10(-6) M) had a biphasic effect on cyclic AMP levels in the skin, an initial increase followed by a decrease. Indomethacin also potentiated prostaglandin E1 stimulated cyclic AMP accumulation. 8. Theophylline increased cyclic AMP levels in the skin and potentiated prostaglandin E1 stimulated cyclic AMP accumulation. 9. Pre-treatment of the skin with theophylline reversed the effects of cyclic AMP on the short-circuit current and open-circuit potential. 10. It is concluded that endogenous prostaglandins help to maintain sodium transport across isolated frog skin and that the effects of E-type prostaglandins on the short-circuit current are mediated by increased cyclic AMP levels. The transient increase in short-circuit current and the increased skin sensitivity caused by indomethacin (6 X 10(-6) M) are attributed to inhibition of phosphodiesterase activity. The failure of theophylline to potentiate the short-circuit current response of the skin to prostaglandin E1 is attributed to alteration of cyclic AMP action on the skin by theophylline.