A resonant bubble detection method based on a second harmonic technique has been used to monitor the femoral vascular system of dogs subjected to rapid decompression. For this study, the detector consisted of two acoustic transducers mounted at right angles to each other that were packaged in a perivascular cuff configuration. This detector responds selectively only to bubbles near resonant size (4.2 mum in diameter); solid particles and large bubbles produce no response. The detector was used to monitor a total of 15 dogs. Eleven dogs were subjected to a series of simulated underwater dives until acute symptoms of decompression sickness occurred; 4 dogs served as controls. In the dived group, either the femoral vein or the femoral artery was monitored. Resonant bubbles were observed in the femoral veins of all 6 dogs monitored at this location. During arterial monitoring, most dogs showed no response, but an occasional weak response was observed in 2 of the dogs. No resonant bubbles were detected in the femoral artery or the femoral vein in any of the controls. The data suggest that this bubble detection method is feasible for in vivo use. Furthermore, 4 mum diameter bubbles are much more prevalent in the veins of dogs suffering from decompression sickness than they are in dog arteries, presumably because they are filtered out effectively by the pulmonary circulation. Modifications of this method are discussed to enhance its accuracy and applicability for quantifying bubble size, location, and number.