Aldosterone is a key component of the renin-angiotensin-aldosterone system, and spironolactone, an aldosterone receptor blocker, shows beneficial effects in patients with end-stage renal disease and heart failure. The aim of the present study was to investigate by means of Ussing chamber technique the effect of spironolactone on the transmesothelial permeability of visceral sheep peritoneum in vitro. Peritoneal samples from the omentum of adult sheep were collected immediately after slaughter in a cooled and oxygenated Krebs-Ringer bicarbonate (KRB) solution. Isolated intact sheets of peritoneum were mounted in an Ussing-type chamber. Spironolactone (10(-5) mol/L) was added apically and basolaterally to the KRB solution. The transmesothelial resistance (R) was measured before and serially for 30 minutes after the addition of the substances. Data present the mean +/- standard error of 6 experiments in each case. The control R was 19.8 +/- 0.36 omega x cm2. The addition of spironolactone resulted in a reduction in the R, which became significant on both sides of the membrane within 10 minutes and remained significantly different thereafter. The maximum reduction of R (deltaR%) reached 24.8% +/- 2.3% (p < 0.01) apically and 26.3% +/- 3.2% (p < 0.01) basolaterally. Our data clearly show that spironolactone increases the permeability of visceral sheep peritoneum in a lasting manner. Increased peritoneal permeability could result in increased sodium removal, which has acknowledged beneficial effects both in patients undergoing peritoneal dialysis and in patients with heart failure. Further clinical studies investigating the effect of spironolactone on sodium removal in peritoneal dialysis are justified.