Three measures of locomotory performance and a series of variables thought to affect performance were measured in the iguanid lizard Ctenosaura similis. Burst speed is mass independent; however, endurance time at 1 km/h (EN-DUR) and maximal distance run (MAX DIS) scale as M0.3. Standard and maximal rates of O2 consumption (VO2max) scale as M0.9; VO2max averages 10-fold greater than standard metabolic rate (SMR). Three of ten enzyme activities measured exhibit significant scaling. After statistically removing the effects of body mass, multiple-regression analysis indicates that 1) 89% of the residual variation in ENDUR is correlated with variation among individuals in thigh muscle mass, VO2max, heart mass, and liver citrate synthase (CS) activity; 2) maximal CO2 consumption (VCO2max) and thigh pyruvate kinase activity statistically explain 64% of the variation in MAX DIS; 3) heart and liver masses together predict 35% of the variation in SMR; 4) thigh and liver CS activity, heart lactate dehydrogenase (LDH) activity, and hematocrit account for 67% of the variation in VO2max;5) 97% of the variation in VCO2max is statistically related to variation in liver CS activity, thigh and heart masses, and heart LDH activity.