Variation in the growth rate of long bones is a function of the number of dividing cells in the columns of the proliferation (flat) cell zone of the growth plate, the frequency with which they divide, and the size to which they grow prior to ossification. In a previous study we found that the wide variation in bone growth rates seen among species of birds was largely associated with variation in the numbers of cells in the flat cell zone. Here we have undertaken a similar study of the growth plates of mammals and have examined variation in the morphology and cell kinetics of the tibial growth plates of a variety of species. The bone growth rates tended to he lower than those observed in birds and were particularly low in the anthropoid primates. Although quite marked variation in flat cell numbers is apparent, the results suggest that variation in cell division rate may play a relatively greater role in variation in bone growth rate among mammals than it does in birds. and that the very low hone growth rates seen in the primates are due, in part, to lower rates of cell division than in other species.