Since recent investigations have shown elevated urinary PGE2 and polyuria in hypokalemic animals which were reversed by PG synthesis inhibition with indomethacin, studies were undertaken to examine the effects of extracellular [K+] on renomedullary PG production in vitro. Slices of rabbit and human renal papilla were incubated in Krebs-Ringer HCO3- buffer, 95% O2-5% CO2, glucose 10 mM, HSA 4 gm/100 ml, for 30 min at 38 degrees C, with and without 1-14C-AA (10 micrometer). Measurments were made of total endogenous iPGE2 and iPGF2alpha production and radioactive AA leads to PGE2. In rabbit renal medulla values for iPGE2 (nmol/gm/30 min) were 252 +/- 20 at [K+] 0; 182 +/- 17 at [K+] 2.5 mEq/L; 163 +/- 18 at [K+] 5.5; and 129 +/- 17 [K+] 9.0 (p less than 0.005). iPGF2alpha was unaltered by changes in media potassium concentrations (6.8 +/- 0.9 nmol/gm/30 min at [K+] 0 and 6.2 +/- 0.8 at [K+] 9.0 MEq/L). In the human renal medulla iPGE2 was 9.5 +/- 1.6 nmol/gm/30 min at [K+] 0; 5.0 +/- 0.7 at [K+] 2.5 mEq/L; 5.3 +/- 0.3 at [K+] 5.5; and 4.6 +/- 1.0 at [K+] 9.0 (p less than 0.05). AA leads to PGE2 (nmol/gm/30 min) was 3.21 +/- 0.92 at [K+] 0; 2.47 +/- 0.57 at [K+] 2.5 mEq/L; 1.30 +/- 0.30 at [K+] 5.5; and 0.76 +/- 0.4 at [K+] 9.0 in rabbit medulla (P less than 0.005). It is postulated that direct stimulation of papillary PGE2 biosynthesis by low extracellular [K+] impairing the cAMP-generating response to vasopressin could represent the initial event in the pathogenesis of vasopressin-resistant polyuria.