In further studying hormonal regulation of microvascular exchange, we tested two hypotheses: 1) atrial natriuretic peptide (ANP) would increase capillary permeability to protein and 2) the actions of ANP on capillary permeability would be independent of the perfusate protein composition. These hypotheses were tested by assessing in situ capillary transport coefficients: hydraulic conductivity (Lp), apparent and diffusive permeabilities (Ps and Psd, respectively) to the protein alpha-lactalbumine, and convective coupling [Lp(1-sigma)] in mesenteric capillaries in cerebrally pithed frogs (Rana pipiens). The transport coefficients were determined in the absence and presence of frog ANP (fANP) during perfusion with either frog plasma or bovine serum albumin (BSA). Control Lp was 1.8-fold greater in vessels perfused with BSA compared with plasma. In the presence of 10 or 100 nM fANP, Lp was increased by approximately 3.5-fold in plasma-perfused vessels and approximately 2-fold in BSA-perfused vessels from their respective controls. Control Ps was 4.2-fold higher in vessels perfused with BSA compared with plasma. Despite the differences in control permeability, the increase in Ps by fANP was of similar magnitude (2- to 3-fold) for both protein perfusates. Analysis of the pressure-dependent alpha-lactalbumin flux suggested that the increase in capillary permeability induced by fANP is consistent with fANP increasing permeability without altering the selectivity of the capillary barrier.