The effect of mechanical stretching on prostacyclin (PGI2) synthesis was studied by growing bovine aortic endothelial cells on flexible-bottomed culture plates that could be deformed by vacuum. A stress unit was used to exert an elongation of 24% at maximum downward deflection of the culture plate bottom. The experimental group was subjected to cycles of 10 seconds of elongation, 10 seconds of relaxation for 1, 3, or 5 days. The control group was subjected to similar incubations but without cyclic stretch. Twenty-four hours before collection, the medium was replaced with new medium that was devoid of serum. On days 1, 3, and 5, the 24-hour culture medium was collected (basal state). Arachidonic acid (20 mumol/L) was then added to each culture and incubated for 5 minutes at 37 degrees C. The medium was then collected to assess prostacyclin synthetic activity (stimulated state). Media were assayed for PGI2 and thromboxane A2 by radioimmunoassays for 6-keto-PGF1 alpha and thromboxane B2, the respective stable hydrolysis product. The results indicate that cyclic stretching, while not altering the basal production of PGI2, increases PGI2 synthetic capacity in a time-dependent manner. These data suggest that it may be inappropriate to extrapolate the mechanisms of in vivo PGI2 release from studies of endothelial cells in stationary culture.