Computer modeling of cardiac propagation suggests that curvature of muscle fibers modulates conduction velocity (CV). The effect could be involved in arrhythmogenesis by altering the dynamics of reentrant wavefronts or by causing propagation block. To verify the existence of this effect experimentally, we measured CV in anisotropic neonatal rat ventricular myocyte monolayers. The orientation of the cells was directed by scratches machined into plastic coverslips. Each substrate contained a region in which scratch radius of curvature varied from 0.25 to 1.0 cm. The CV anisotropy ratio (longitudinal CV/transverse CV in straight fiber regions) was 2.3 ± 0.3 (n = 38). We initiated wavefronts transverse to fibers with the fibers either curving toward or away from the wavefronts. Action potentials were recorded using a potentiometric dye and a video camera. Propagation was faster (p =0.0003) when fibers curved toward wavefronts than when fibers curved in the opposite direction. The mean CV difference was 0.38 ± 0.44 cm/s (± n= 24), which is 3.5% of nominal straight fiber transverse CV (11.0 ± 3.2 cm/s). The effect was also present (p =0.07) when pacing was slowed from 350 to 500 ms (n= 6). In a control group (n= 8) with uncurved fibers, CV was the same in both directions(p= NS). We conclude that fiber curvature is a factor in modulating cardiac propagation. © 2009 IEEE.