OBJECTIVE: To characterize regional capillary flow direction in the isolated perfused heart subjected to acute hypoxia. DESIGN: A technique incorporating sequential infusion of differently coloured 10 microns microspheres into hearts of adult male Sprague-Dawley rats was employed in two series of experiments. In series A, a control group (n = 10) received three infusions of coloured microspheres (each distinctly coloured) in sequence during normoxic perfusion. A 'hypoxic' group (n = 12) received the initial two infusions under identical conditions, with the third infusion under conditions of experimental hypoxia. Hearts in series B (n = 5) received two differently coloured infusions, the initial under normoxic conditions and the second during hypoxia. Subsequent histological examination of 40 microns sections obtained from midmyocardial (MID) and subendocardial (SUB-E) regions of the heart revealed flow vectors based on the sequence of colours trapped within neighbouring capillaries. RESULTS: Series A-A prevalence in concurrent flow was observed among neighbouring capillaries, with significantly lower percentages in SUB-E than in MID (P < 0.01). A higher percentage of countercurrent flow was revealed subsequent to the hypoxic intervention than during the earlier normoxic period, as well as in comparison with normally perfused controls (P < 0.01). Flow vector observations from series B depicted lower proportions of concurrent flow compared with the two-colour normoxic interval in 'hypoxic' hearts from series A (P < 0.05). CONCLUSION: Reversals in capillary flow direction, suggested by the reorientation of final injected microspheres relative to the positions of earlier injected microspheres, were interpreted as a response to sustain myocyte oxygenation due to severe reductions in arterial oxygen tension.