The heart mitochondrial properties and the aerobic capacity (VO2max) of the rat (Sprague-Dawley breed) and the Cuban iguana (Cyclura nubila) were used to evaluate the relationship between the oxidative capacity of the heart and the maximum oxygen delivery rate. Both species are active at body temperatures of 37-39 degrees C, have similar heart mitochondrial volumes [Vmt; 0.43 +/- 0.02 ml (S.E.M.) in the rat and 0.48 +/- 0.02 ml in the iguana] and differ less than twofold in VO2max (29.2 +/- 1.6 and 16.9 +/- 0.6 ml min-1, respectively). We found that Vmt was closely correlated with VO2max in the rat (r2 = 0.77, P < 0.005) and the iguana (r2 = 0.82; P < 0.001). Furthermore, the inner mitochondrial membrane (cristae) area (Sim) per unit VO2max did not differ between the rat and the iguana (0.60 +/- 0.02 and 0.71 +/- 0.02 m2 min ml-1 O2, respectively). This correspondence of Sim/VO2max indicates that the rat and the iguana have the same cardiac oxidative capacity at the maximum oxygen delivery rate. These results suggest that, despite the differences between the cardiovascular systems of these species, the cardiac cost of delivering oxygen at the aerobic capacity is similar in this mammal and this reptile.