In conscious Wistar-Kyoto rats, we studied the uptake of radioactive tracer ¹²⁵I-albumin into the pleural space and circulation after intraperitoneal (IP) injections with 1 or 5 ml Ringer solution (3 g/dl albumin). Postmortem, we sampled pleural liquid, peritoneal liquid and blood plasma 3-48 h after intraperitoneal (IP) injection and measured their radioactivity and protein concentration. Tracer concentration was greater in pleural liquid than in plasma ~3 h after injection with both IP injection volumes. This behavior indicated transport of tracer through the diaphragm into the pleural space. A dynamic analysis of the tracer uptake with 5 ml IP injections showed that at least 50% of the total pleural flow was via the diaphragm. A similar estimate was derived from an analysis of total protein concentrations. Both estimates were based on restricted pleural capillary filtration and unrestricted transdiaphragmatic transport. The 5 ml IP injections did not change plasma protein concentration but increased pleural and peritoneal protein concentrations from control values by 22% and 30%, respectively. These changes were consistent with a small (~8%) increase in capillary filtration and a small (~20%) reduction in transdiaphragmatic flow from control values, consistent with the small (3%) decrease in hydration measured in diaphragm muscle. Thus, the pleural uptake of tracer via the diaphragm with the IP injections occurred by the near normal transport of liquid and protein.