The effect of diazoxide and glucagon were studied on the freely draining upper urinary tract and after varying periods of obstruction in dogs. Without obstruction, diazoxide produced longer inactive periods than glucagon and recovery was associated with less retrograde activity. Following obstruction for 5–10 min, 6 and 24 h, diazoxide decreased renal pressure and the effect was greatest when the upper tract was obstructed for the shortest period. Glucagon also produced an overall reduction in pressure after 5–10-min and 6-hour periods of obstruction, but there was an initial rise in pressure after 5–10 min of obstruction. At 24 h there was an increase, but no overall decrease in pressure after glucagon. Diazoxide produced a greater fall in pressure after 5–10 min and 24 h of obstruction, and a faster fall in pressure after 5–10 min and 6 h of obstruction as compared to glucagon. Diazoxide may be a more effective therapeutic agent in the management of ureteral calculus disease than glucagon.