This study used cross-sectional (one observation per each of several individuals of different size) and longitudinal (more than one observation through time per individual) methods to determine the effects of size on speed and endurance of Thamnophis sirtalis fitchi. The cross-sectional sample consisted of 497 snakes from a single population. Log of mean burst speed in this group measured over 50 cm (V50) was a quadratic function of log of snout-vent length (SVL); the slope of this polynomial ranged from about 1.6 to -0.3 for the smallest to the largest snakes (maximal V50 at 50.6 cm SVL). Longitudinal measurements of log V50 also were quadratically related to log SVL, but the slopes varied depending on year in which performance was measured. Cross-sectional allometry revealed that the slope of the regression relating log(endurance) to log(SVL) was about 2.3, and longitudinal estimates of this quantity ranged from 0.5 to 3.2, depending on the year. Sex did not affect burst speed, but females had significantly less endurance than males of equal SVL, and pregnancy had a significant detrimental effect on both speed and endurance. Regressions with SVL as the independent variable were used to generate size-corrected (residual) values of speed, endurance and mass, and each of these residuals had significant repeatability. For example, during 1986 the short-term repeatabilities (Pearson's r) of speed and endurance residuals were 0.65 (P < 0.001) and 0.57 (P < 0.001). From 1986 to 1987, year-to-year repeatabilities of speed, endurance and mass residuals were 0.25 (P= 0.001), 0.22 (P= 0.005) and 0.47 (P < 0.001), respectively. Analysis of these three respective residual values of 264 neonatal snakes from 34 litters revealed highly significant percent variance components attributable to litter of 14%, 34% and 36%, yielding respective heritabilities of 0.28, 0.68 and 0.72. Speed and endurance residuals had a low but significant positive correlation (r= 0.26, n= 497), due apparently to snakes with poor performance: high speed is not linked with high endurance. A squared value of the mass residual had a significant negative correlation with both speed residual (r= -0.105, n= 497) and endurance residual (-0.24), suggesting that snakes deviating from a mean value of mass relative to length have a slight decrement in locomotor performance. Longitudinal estimates of the scaling of mass with length revealed significant variation associated with different ages of snakes and different years of the study that could not be obtained from cross-sectional data.