The present article tests one assumption of the thermogenesis hypothesis of cetacean brain evolution (Biol. Rev., Vol. 81, pp. 293-338, 2006) by analysing phylogenetically correct correlations between relative brain mass and environmental temperatures in 20 odontocetes, as well as undertaking a phylogenetic analysis of covariance of differences between the clades in brain mass. A simulation of trait evolution, under an Ornstein-Uhlenbeck process, yielded phylogenetically correct allometric slopes, which were used to correct the body size in correlations and ANCOVAs; the simulation also produced null distributions for these analyses. No significant correlation was found between environmental temperatures and relative brain masses, and neither was there a significant difference between clades in brain masses. These results challenge one of the central assumptions of the thermogenesis hypothesis, but other assumptions from this hypothesis are still to be explained. Alternative hypotheses are also discussed, but further studies will be necessary to judge their validity.