Animal studies have shown increased fluid absorption from the peritoneal cavity following intraperitoneal (ip) vasopressin. Lithium is known to antagonize vasopressin effects on fluid absorption in kidney distal nephrons. The aim of the present study was to see whether lithium-containing exchanges increase the ultrafiltration rates (UF) during peritoneal dialysis (PD) in rats. PD was carried out in 6 Sprague-Dawley rats with 1.5% dextrose-containing PD solution using 15-ml volumes. Each exchange (ex) took 1 min for inflow, 4 mins for outflow and 25 mins for dwell. All rats underwent 9 consecutive half-hourly exs. During exs 4-6 lithium carbonate 2.5 mM was added to the PD solution. During lithium-containing exs significant increases in the glucose absorption rates (3.9 +/- 7.8 vs 37.5 +/- 8.1 mg/ex; p = 0.025) were associated with significant reductions in the UF (3.03 +/- 0.25 vs 1.78 +/- 0.12 ml/ex; p = 0.005). In conclusion, the isolated increase in glucose absorption without increases in the dialysate protein concentration with ip lithium, may suggest either a selective increase in size of the pores with a mean dimater near that of the glucose molecule or enhanced lymphatic absorption. ip lithium did not increase the UF in a rat model of PD.