The effect of various steroids and prostaglandins on the activity of canine kidney Na+, K+-ATPase activity was tested in vitro. Progesterone induced a dose-dependent increase in inhibition of the canine kidney Na+, K+-ATPase activity. Aldosterone, cortisol, cortisone as well as prostaglandin E2 and 13,14-dihydro-15-keto-prostaglandin F2 alpha, did not cause an inhibition of canine kidney Na+, K+-ATPase.