While it is well established that the lymphatic vasculature is central to fluid and solute homeostasis, how it accomplishes this task is not well defined. To clarify the basic mechanisms underlying basal fluid and solute homeostasis, we assessed permeability to rat serum albumin (P(RSA)(s)) in mesenteric collecting lymphatic vessels and venules of juvenile male rats. Using the quantitative microfluorometric technique originally developed for blood capillaries, we tested the hypothesis that as a consequence of venules and collecting lymphatics sharing a common embryological origin, their P(RSA)(s) would not differ significantly. Supporting our hypothesis, the median collecting lymphatic P(RSA)(s) (3.5 +/- 1.0 x 10(7) cm s(-1), N = 22) did not differ significantly from the median venular P(RSA)(s) (4.0 +/- 1.0 x 10(7) cm s(-1), N = 8, P = 0.61). For collecting lymphatics the diffusive permeability (P(d) = 2.5 x 10(7) cm s(-1)) was obtained from the relationship of apparent P(RSA)(s) and pressure. While the measured P(RSA)(s), P(d) and estimated hydraulic conductivity of collecting lymphatics and venules were similar, the contribution of convective coupling differs as a result of the higher hydrostatic pressure experienced by venules relative to collecting lymphatics in vivo. In summary, the data demonstrate the capacity for collecting lymphatics to act as exchange vessels, able to extravasate solute and filter fluid. As a consequence these data provide experimental support for the theory that prenodal lymphatic vessels concentrate intraluminal protein.