Two heme-peptides (HP) of about 20-A diameter (heme-undecapeptide [HI IP], mol wt ~ 1900 and heme-octapeptide [H8P], mol wt ~1550), obtained by enzymic hydrolysis of cytochrome c, were used as probe molecules in muscle capillaries (rat diaphragm). They were localized in situ by a peroxidase reaction, enhanced by the addition of imidazole to the incubation medium. Chromatography of plasma samples showed that HPs circulate predominantly as monomers for the duration of the experiments and are bound by aldehyde fixatives to plasma proteins to the extent of ~50% (H8P) to ~95% (HI IP). Both tracers cross the endothelium primarily via plasmalemmal vesicles which become progressively labeled (by reaction product) from the blood front to the tissue front of the endothelium, in three successive resolvable phases. By the end of each phase the extent of labeling reaches > 90% of the corresponding vesicle population. Labeled vesicles appear as either isolated units or chains which form patent channels across the endothelium. The patency of these channels was checked by specimen tilting and graphic analysis of their images. No evidence was found for early or preferential marking of the intercellular junctions and spaces by reaction product. It is concluded that the channels are the most likely candidate for structural equivalents of the small pores of the capillary wall since they are continuous, water-filled passages, and are provided with one or more strictures of < 100 A. Their frequency remains to be established by future work.