Mass transfer area coefficients (PS) to small solutes are usually markedly increased during the first 0–15 min of peritoneal dialysis (PD) dwells. This phenomenon may be due to, for example, initial arteriolar vasodilatation and, hence, recruitment of capillary surface area. Other possibilities are an initial discharge (or saturation) of solutes from (in) the interstitium or an increased mixing, i.e., ‘macrostirring’ caused by the exchange procedure per se. We have investigated these possibilities during acute PD in rats, by assessing PS for <sup>51</sup>Cr-EDTA as a function of time [PS(t)]. The discharge effect was studied by saturating the peritoneal interstitium with <sup>51</sup>Cr-EDTA by intravenous tracer infusion prior to each dwell and the results compared to those obtained in rats when tracer infusion and dwells were started simultaneously. The potential effect of initial vasodilatation was studied by adding isoproterenol to the dialysis fluid. Finally, the potential influence of an increased interstitial ‘microstirring’, induced by high glucose concentrations, was investigated by comparison of results for 1.36% Dianeal<sup>®</sup> with those for 3.86% Dianeal. In nonvasodilated rats there was a significant drop in PS(t) between 2.5 and 15 min regardless of whether the rats were preloaded with tracer or not. However, there were no significant changes in PS(t) within the isoproterenol-treated group, indicating that vasodilatation plays a crucial role for the high PS initially. Furthermore, there was no difference in overall PS for <sup>51</sup>Cr-EDTA among 1.36 and 3.86% Dianeal dwells. In conclusion, we have found that vasodilatation, but not interstitial discharge (or loading), may explain the inflation of PS occurring during the initial part of PD dwells. In addition, ‘macrostirring’, induced by the exchange procedure per se, may also be important.