Awake, unrestrained rats with chronic implants of a Doppler probe on the vena cava, were saturated in a small plexiglass chamber at a maximum pressure (P1), rapidly decompressed to a predetermined pressure (P2) and then monitored for the presence of intravascular bubbles. If there were no indication of bubbles within 1 h, the decompression was considered bubble free. For each saturation pressure, an increasing pressure difference (deltaP = P1 - P2) was tried on each exposure until the threshold for bubble detection was found. The relationship deltaP = a P1 + b, where a and b are constants defining the threshold for bubble detection, was experimentally determined at pressures from 3-10 ATA. The depth-dependent linear relationship deltaP = 0.38P, + 1.29 (r = 0.97, P is less than 0.001) was found for first exposures and deltaP = 0.62 P1 + 0.16 (r = 0.99, p is less than 0.001) for repeat exposures. The chronic Doppler preparation allows a more sensitive threshold determination than past methods for small animals and also allows repeat pressure exposures without the effects of a severe decompression stress.