Intravitreal injections of kainic acid were used to examine the significance of normal retinal activity for eye growth in chickens, this acid being chosen because of its known, selective neurotoxic effects on cells in the chicken retina. A 6 nmole dose of kainic significantly reduces amacrine cell numbers when used in very young chickens, while higher doses of kainic acid also affect bipolar and horizontal cell numbers. The effects of intravitreal injection of kainic acid on eye growth were assessed 4 weeks after treatment. A 200 nmole dose of kainic acid, used with day-old and 14-day-old chickens, had opposing effects on the anterior and posterior segments of the eye; while growth of the anterior segment was inhibited, the posterior segment was enlarged, predominantly in the equatorial direction. A 20 nmole dose of this acid similarly affected growth in 14-day-old chickens, but in day-old chickens, the anterior segment was also enlarged and the overall eye enlargement had an axial bias. Myopia was the most common refractive error associated with both patterns of development. A 2 nmole dose of kainic acid was without effect on eye growth. Parallels are drawn between these eye enlargement phenomena and those described in chickens whose visual environments have been manipulated. Our results indicate that normal retinal activity is fundamental to normal eye growth in chickens, and furthermore, that growth of the anterior and posterior segments of the chicken eye are independently regulated.