The number of motoneurons in the lumbar lateral motor column (LMC) was compared in bullfrogs (Rana catesbeiana) ranging in body length from 2.5 to 19 cm. Large frogs had 36% more motoneurons than small frogs; however, within the caudal third of the LMC, large frogs had over 70% more motoneurons than small frogs. Injection of small frogs with [3H]thymidine every third day for 20-22 weeks gave no evidence of motoneuron birth. Instead, a pool of small, incompletely differentiated (type L) motoneurons appears to be converted into mature (type M) motoneurons as the animal grows. This hypothesis is supported by several lines of evidence: (1) the number of type-M motoneurons varies directly with body size while the number of type-L cells varies inversely; (2) the increase in type-M motoneurons and the decrease in type-L cells are restricted to the same regions of the LMC; and (3) type-L cells exhibited both immunoreactivity to neurofilament antibodies and histochemical evidence of acetylcholinesterase activity, a marker for spinal motoneurons.