A small ring chamber (I.D. = 6 mm) was placed on the exposed lung of anaesthetized bullfrogs. A localized hypoxia was induced in the ring chamber by introducing nitrogen in it. Blood flow velocity in pulmonary microvessels was measured by means of a laser Doppler microscope. The mean blood flow velocity was 1.98 +/- 0.45 and 1.52 +/- 0.10 mm/sec during the control condition in arterioles and capillaries, respectively. It was then reduced by the localized hypoxia to 1.63 +/- 0.32 and 1.33 +/- 0.08 mm/sec in arterioles and capillaries, respectively. The reduction, when expressed in the percentage ratio to the control flow velocity in each blood vessel group, was significantly larger in arterioles than in capillaries. A phase delay in the pulsation of the flow velocity contour was detected only in arterioles. These differences between pulmonary arterioles and capillaries in response to the localized hypoxia may be attributed to the dense interconnection of capillary network extending beyond the localized hypoxic area to the normoxic area.