Despite its relevance to the physiology of lymph formation and propulsion, the instantaneous flow velocity in single lymphatic capillaries has not been measured to date. The method of fluorescence recovery after photobleaching (FRAP) was adapted for this purpose and used to characterize flow in the lymphatic capillaries in tail skin of anesthetized mice during a constant-pressure intradermal injection of fluorescein isothiocyanate-dextran (mol wt 2 x 10(6). The median lymph flow velocity was 4.7 microns/s, and the velocity magnitude ranged from 0 to 29 microns/s. The direction of flow was generally proximal, but stasis and backflow toward the site of injection was also detected. Evidence for oscillatory flow was detected in some FRAP experiments, and in separate experiments a periodicity of approximately 120 min-1, directly correlated to respiration frequency, was measured by tracking the motion of fluorescent latex microspheres (1 micron diam) introduced into the lymphatic capillary network. The velocity magnitude showed a correlation with duration of infusion but not with distance from injection site. It is speculated that the temporal decay of mean velocity magnitude could be related to the relaxation of local pressure gradients as partially collapsed vessels expand during the infusion.