To study the effects of ADH on the transport of potassium by the distal tubule and collecting duct system, we performed simultaneous clearance and micropuncture experiments in homozygous Brattleboro rats (with hereditary hypothalamic diabetes insipidus), before and after intravenous infusion of the hormone. Final urinary flow rate was reduced by a factor of 7 after ADH, but fractional potassium excretion increased by 77% for the group as a whole. During free-flow micropuncture, there was no significant difference in fractional delivery of potassium up to the late distal tubule between control (water diuresis) and ADH conditions; thus, the increase in final urinary potassium excretion was mediated beyond this tubular site. However, flow rate of tubular fluid was decreased significantly after ADH in late distal tubular segments, where potassium secretion is a flow-dependent process. To evaluate the possibility of a direct effect of ADH on distal tubular potassium secretion, independent of changes in flow rate, we studied another group of animals by continuous microperfusion, in vivo, of single distal tubules, using an isotonic perfusion fluid so that water reabsorption would be minimal after as well as before the addition of ADH. Under these conditions, a significant stimulation of distal tubular potassium secretion by ADH could be demonstrated. We suggest that this property of ADH may serve to prevent potassium retention during periods of antidiuresis.