1. The theory of steady-state filtration through capillary walls (Michel, 1984) has been developed and investigated in experiments on single capillaries of the frog mesentery perfused with Ringer solutions containing bovine serum albumin (BSA) and Ficoll 70. 2. In each experiment, the micro-occlusion technique of Michel, Mason, Curry, Tooke & Hunter (1974) has been used to investigate the relation between fluid movements per unit area of capillary wall (Jv/A) and capillary pressure (Pc) under two sets of conditions in the same vessel. First, the relation has been determined following brief perfusions, where the difference in oncotic pressure across the capillary wall approximated to the perfusate oncotic pressure at all values of Pc. These results are referred to as the transient data. Secondly, the relation was investigated by estimating Jv/A at values of Pc which had been maintained constant during at least 2 min of perfusion prior to the measurement. Under these conditions the concentration of macromolecules in the pericapillary fluid was determined by the steady-state composition of the filtrate passing through the capillary wall, and these results are referred to as steady-state data. 3. In all fifteen capillaries investigated, the relationship between Jv/A and Pc was linear for the transient data and conspicuously non-linear in the steady state. When Pc exceeded the oncotic pressure of the perfusate, steady-state values for Jv/A lay slightly above but parallel to the transient values for the same vessel. At values of Pc less than the perfusate oncotic pressure, the transient data showed reabsorption of fluid from the tissues, but in the steady state either fluid movements were so small as to be undetected or slight filtration was observed. The steady-state data followed the pattern predicted by theory. 4. The transient data were used to estimate the reflection coefficient of the capillary wall (sigma) to the macromolecular solute. In seven vessels, the mean sigma to BSA was 0.76 (S.E. of mean +/- 0.04) and in eight different vessels mean sigma to Ficoll 70 in the presence of BSA (10 mg ml-1) was 0.98 (S.E. of mean +/- 0.05). The steady-state data were consistent with the prediction that the oncotic pressure opposing high filtration rates approximates to sigma 2 pi c in the steady state, where pi c is the perfusate oncotic pressure.(ABSTRACT TRUNCATED AT 400 WORDS)