Low-frequency (4-12 cpm) spontaneous oscillations in cerebral blood flow are well known and their augmentation after nitric oxide synthase inhibition has recently been described. However, the presence of these oscillations in blood flow velocity in the capillary network of the brain has not been demonstrated. In this paper, low-frequency oscillations in red blood cell flow velocity in cortical capillaries using intravital video microscopy were studied before and after infusion of the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME). Fluorescein isothiocyanate-labeled red blood cells were injected intravenously and served as markers of capillary flow. Red cell velocity was measured by off-line image tracking. After infusion of L-NAME (30 mg/kg), red cell velocity decreased from an average of 1.0 +/- 0.1 mm/sec to 0.53 +/- 0.1 mm/sec. Simultaneously, low-frequency oscillations in velocity emerged as indicated by an 81 +/- 17% increase in standard deviation of the 4- to 8-cpm frequency components. There was a significant temporal correlation (r = 0.58, P < 0.01) in red cell velocity between neighboring capillaries after L-NAME. Principal component analysis suggested that the high temporal correlation was a consequence of low-frequency oscillations rather than phase. These results are consistent with a model in which low-frequency spontaneous oscillations in flow velocity are brought about by precapillary vasomotion. This study provides for the first time direct evidence for low-frequency synchronous oscillations of red cell flow velocity in the cerebral capillary network.