We hypothesized that during chick embryo cardiac development, cycle length decrease (heart rate increase) may be associated with developmental changes in the ventricular relaxation rate constant, tau, which can be estimated from ventricular pressure decrease by the function P(t) = P0e-t/tau, where P0 = pressure at the time of minimum time derivative of ventricular pressure and t = time. Natural logarithm conversion results in a linear relation between In P(t) and t with slope of -1/tau. We determined tau in Hamburger-Hamilton stage 17-27 (d 3-5) embryos (n = 35) at intrinsic cycle length and in stage 24 embryos (n = 5) during cardiac cycle length perturbations with the hot-cold probe technique. Regression analysis of In P(t) from the minimum dP/dt until t = 30 ms was used to estimate tau. Intrinsic cycle length decreased during development from stage 17 to 27 and tau decreased linearly as cycle length decreased (r = 0.39, p less than 0.005). Thus, during early development, both tau and cycle length decrease (relaxation rate and heart rate increase). In stage 24 embryos, cycle length perturbation yielded a nonlinear cycle length-dependent relation with tau, with a plateau at cycle lengths less than intrinsic, i.e. tau decreased with cycle length decrease to intrinsic cycle length, then tau plateaued with further cycle length decrease. These findings demonstrate that ventricular relaxation rate is both maturation and cycle length dependent. The plateau effect of the tau-cycle length relationship may be due to developmental limitations of the calcium transport system.