Insulin secreting (beta) cell and pancreas volumes were estimated from bullfrogs at different stages in the life cycle. Estimations were made after immunostaining and planimetry of serial pancreas sections. The beta cell volume increased during larval life and is the most likely source of the increased insulin-like immunoreactivity extracted from tadpole pancreases (J. Hulsebus and E. S. Farrar (1985) Gen. Comp. Endocrinol. 58, 114-119). Metamorphic climax was associated with a loss of pancreas and beta cell volumes and insulin-like immunoreactivity. The beta cell pancreatic volume increased following metamorphosis and growth continued in juveniles and adults. The beta cell mass formed a relatively constant percentage of the total pancreatic volume throughout the life cycle.