We have studied some characteristics of forskolin (FSK) actions on osmotic water flow in frog urinary bladder. FSK, in fact, when used at concentrations higher than 0.1 μM, directly stimulates osmotic water flow, whereas at concentrations lower than 0.05 μM it acts synergistically with submaximal doses of vasopressin (ADH) in regulating osmotic water flow to analyze whether both FSK actions are modulated by the activity state of Ns, we have examined the effect of GTP or GDP specific analogs. The results obtained indicate that the stimulatory and potentiative actions of FSK are modulated to the same extent by the activity state of Ns protein. Moreover the preincubation with cycloheximide, a protein synthesis inhibitor, significantly reduces both FSK-actions leaving the osmotic water flow induced by ADH unchanged: this indicates that the sites of action of FSK on osmotic water flow may reside on a protein with a relatively shorter half-life than other components of adenylate cyclase.