The effects of flow and flow changes on arterial diameter were investigated in vitro on isolated rat femoral arteries. Segments of femoral arteries were excised, mounted on microcannulas, and perfused with Tyrode's solution (37 degrees C). Perfusion pressure was kept constant at 90 mm Hg. The mean external diameter after equilibration at a transmural pressure of 90 mm Hg was 720 +/- 50 microm (n = 12). Vessels were then constricted with norepinephrine (1 microM in the superfusion solution) to 77% +/- 13% of the resting diameter; acetylcholine was used to check endothelial function. The external diameter was measured continuously using video microscopy. The arteries were subjected to two different types of flow variations: (a) step changes in flow (increase and decrease, n = 6) and (b) low-frequency sinusoidal flow variations (frequencies ranging from 0.002 to 0.1 Hz, n = 11). Flow ranged from 0 to 800 microl/min (shear stress ranging from 0 to 15 dyn/cm2). All measured vessels constricted as flow increased. Flow steps induced exponential-like contractions (flow increase) or relaxations (flow decrease) with mean characteristic time constants 31 +/- 4 and 22 +/- 2 s, respectively. Sinusoidal flow oscillations induced sinusoidal diameter oscillations with a time delay. An increase in the frequency of the flow led to a decrease of both the amplitude of the flow-induced diameter oscillations and the phase shift between flow and diameter. The dynamic diameter response to flow changes could be characterized by a first-order low-pass filter with a time constant of 22 s.