Hennemann (1983) provided empirical support for McNab's (1980) hypothesis that a higher specific metabolic rate (SMR) in mammals translates into a higher intrinsic rate of increase (r m ). However, the few marine mammals in Hennemann's data base were excluded from any detailed analyses because their "high rates of metabolism but only average or low values of r m " (p. 106) were thought to reflect trade-offs between maintenance and production necessary to compensate for heat loss in aquatic environments (Hennemann 1983, also see McNab 1980).To investigate further the relationships among r m , body size, and specific metabolic rate in marine mammals (pinnepeds, sirenians, and cetaceans), r m was estimated for 37 populations using published life-history data and Cole's (1954) equation (Hennemann 1983). Estimates of r m in relation to body size in marine mammals were generally within the 95% confidence limits calculated for terrestrial mammals using Hennemann's data. Contrary to Hennemann's (1983) observations, eight of these populations had an r m which was higher in relation to body size than predicted by the average terrestrial mammalian relationship. Furthermore, for marine mammal populations where suitable data were available, r m was correlated with specific metabolic rate (r=0.85, P≦0.035) and all the estimates were again within the 95% confidence limits established from data for terrestrial mammals (Hennemann 1983). It is premature, therefore, to reject the hypothesis that marine mammals do not differ significantly from terrestrial mammals in their allocation of energy for maintanance and reproduction.