Optic fibers from both eyes were made to regenerate simultaneously into one tectum in goldfish. Autoradiography at various times later revealed that regenerating left and right fibers overlapped extensively for up to 4 weeks and subsequently segregated into eye dominance columns by 8 weeks. Continuous tetrodotoxin treatment of both eyes prevented the formation of columns for up to 13 weeks and was equally effective if begun at 6 weeks. When tetrodotoxin treatment was stopped after 11 weeks, columns subsequently formed in the next 3 weeks. Blockade of only one eye did prevent column formation at 8 weeks.