Because the rat papilla has parathyroid hormone (PTH)-sensitive adenylate cyclase and because of indirect evidence that PTH may alter collecting duct water and also calcium transport, the effects of PTH on rat papillary collecting duct water and calcium transport have been studied. PTH in concentrations of 50 and 500 ng/ml significantly increased diffusional water permeability by 20 and 38%, respectively, while 5,000 ng/ml had no additional effect. This permeability response was small when compared to a 78% increase in water permeability with a maximal (0.5 ng/ml) concentration of antidiuretic hormone (ADH). The normal increase in water permeability with ADH was depressed in the presence of PTH (500 ng/ml) but was overcome when the ADH concentration was increased from 0.5 to 5 ng/ml. Neither PTH nor ADH altered the permeability of the collecting duct to calcium which was low (0.19 +/- 0.03 micron/s). Increasing either the bath or perfusate calcium concentration from 1 to 5 mM did not alter calcium permeability. These studies suggest that PTH acts as a partial agonist to ADH within the papillary collecting duct and that PTH is unlikely to have a major role in collecting duct calcium transport.