We estimated synonymous (dN) and nonsynonymous (dS) substitution rates for protein-coding genes of the mitochondrial genome from two individuals each of the species human, chimpanzee, and gorilla. The genes were analyzed both separately and in a combined data set. Pairwise sequence comparisons suggest that the dN/dS rate ratios are about 5-10 times higher in within-species comparisons than in between-species comparisons. This result is confirmed by a more rigorous likelihood ratio test, which rejected the null hypothesis that the dN/dS rate ratios are identical within and between species. The likelihood models account for the genetic code structure, transition/transversion rate ratio, and codon usage bias and are expected to produce more reliable results than the commonly used contingency test. Separate analyses of different genes show that the dN/dS rate ratios are higher within species than between species for all 13 mitochondrial genes, with the difference being statistically significant for all except three small or slowly evolving genes. Furthermore, in conserved genes, nonsynonymous rates within species tend to be higher than the between-species rates by a greater proportion than in fast-changing genes. Our findings confirm and extend earlier results obtained from smaller data sets and suggest the operation of slightly deleterious mutations throughout the mitochondrial genome in the hominoids. Implications of the results for evolutionary studies and, in particular, for studies of the origin of modern humans, are discussed.