The relationship between previously published values of "the intrinsic rate of natural increase" (r m ) and body weight is studied. When organisms covering a wide range of body weights are compared, a correlation is found which can be described by the equation r m =aW n where r m is the intrinsic rate of natural increase per day and W is the average body weight in grams; a is a constant which takes three different values for unicellular organisms and heterotherm and homoiotherm animals respectively. The constant n has a value of about-0.275 for all three groups. This result is compared to the previously found relationship between the metabolic rate per unit weight and body size. It is shown that r m can be inter-preted as the productivity of an exponentially growing population and thus must correlate with metabolic rate. The values of the constants a and n, however, show that for each of the three groups, unicellular organisms, heterotherms and homoiotherms the ratio of energy used for maintenance to that used for production increases with increasing body size and that the evolution from protozoa to metazoa and the evolution from heterotherm to homoiotherm animals in both cases resulted in not only an increased metabolic rate, as shown previously, but also in a decreased population growth efficiency. It is shown that the increase in reproductive potential of homoiotherms relative to that of heterotherms is due to a shorter prereproductive period in the former group.Previous estimates of r m for different species and comparisons between these values in relation to their ecology are discussed in context with the found "r m -body weight" relationship. Attempts to show that such comparisons will be more meaningful when body size is included in the considerations are made. It is suggested that the found relationship may represent the maximum values r m can take rather than average values for all species, since it is likely that the species used for laboratory population-studies are biased in favor of species with high reproductive potentials.