The commonly used anesthetic agent isoflurane (ISO) is a potent coronary vasodilator that could potentially be used in the assessment of coronary reserve, but its effects on coronary blood flow in mice are unknown. Coronary reserve is reduced by age, coronary artery disease and other cardiac pathologies in man, and some of these conditions can now be modeled in mice. Accordingly, we used Doppler ultrasound to measure coronary flow velocity in mice anesthetized with low (1%) and high (2.5%) levels of ISO to generate baseline (B) and elevated hyperemic (H) coronary flows, respectively. A 20-MHz Doppler probe was mounted in a micromanipulator and pointed trans-thoracically toward the origin of the left main coronary arteries of 10 6-wk (Young [Y]), 10 2-y (Old [O]) and 20 2-y apolipoprotein-E null (ApoE(-/-)) atherosclerotic (A) mice. In each mouse, we measured (B) and (H) peak diastolic velocities. B was 35.4 +/- 1.4 cm/s (Y), 24.8 +/- 1.6 (O) and 51.7 +/- 6.4 (A); H was 83.5 +/- 1.3 (Y), 86.5 +/- 1.9 (O) and 120 +/- 16.9 (A) and H/B was 2.4 +/- 0.1 (Y), 3.6 +/- 0.2 (O) and 2.5 +/- 0.2 (A). The differences in baseline velocities and H/B between O and Y and between A and O were significant (p < 0.01), whereas the differences in hyperemic velocities were not (p > 0.05). H/B was higher in old mice as a result of decreased baseline flow rather than increased hyperemic flow velocity. In contrast, ApoE(-/-) mice have increased baseline and hyperemic velocities, perhaps because of coronary lesions. The differences in baseline velocities between young and old mice could be the result of age-related changes in basal metabolism or to differential sensitivity to isoflurane. We conclude that Doppler ultrasound combined with coronary vasodilation via isoflurane could provide a convenient and noninvasive method to estimate coronary reserve in mice, but also that care must be taken when assessing coronary flow in mice under isoflurane anesthesia because of its potent coronary vasodilator properties.