We have estimated from electron micrographs the numbers of axons in the optic nerves of a series of albino rats at 8 different ages ranging from embryonic day (E)18 through postnatal day 28. The number of axons was found to reach a maximum of about 325,000 (324,790 +/- 38,589) on E20, and to decline to about 275,000 (273,744 +/- 20,973) by the day of birth. By the middle of the second postnatal week, the number was further reduced to a stable figure of just over 100,000 (105,809 +/- 7,610). This represents a loss of two-thirds of the axons from the peak value at E20, and suggests that there is a comparable degree of cell death among the retinal ganglion cells. The reduction in the number of optic nerve fibers is not affected significantly by the removal of the opposite eye at birth.