Fluid movement into the peritoneal cavity results after instillation of a hypertonic solution. Some investigators have assumed that the peritoneum is a significant barrier to small solutes and have predicted that fluid would be drawn by an osmotic gradient into the cavity from the tissue surrounding the peritoneal cavity, resulting in tissue hydrostatic pressures well below atmospheric pressure. Contrary to this, we have previously shown that protein and fluid cross the peritoneum and enter the tissue at the same rate during either isotonic or hypertonic dialysis. To investigate the nature of the osmotic barrier of the peritoneum, the hydrostatic pressure profiles were measured in the abdominal wall of the rat during conditions of either isotonicity or hypertonicity in the peritoneal cavity and constant intraperitoneal hydrostatic pressure (Pip). Measurements were made with a micropipette mounted on a micromanipulator and connected to a servo-null pressure measurement system. No interstitial pressures below atmospheric pressure were observed with either type of solution in the peritoneal cavity. For the three Pip values tested, there were few significant differences between the corresponding pressure profiles of isotonic or hypertonic solutions. It is concluded that the parietal peritoneum is not a functional barrier to small solutes, which are often used to raise the osmolality of intraperitoneal solutions. This finding also implies that the tissue interstitium underlying the parietal peritoneum is not the source of water flow into the cavity, which is observed during hypertonic dialysis.