Purpose:To investigate the mechanism and sequence of formation of ring-shaped mitochondria in retinal pigment epithelial cells of a chick model of gyrate atrophy.Methods: Electron microscopic analysis of the ultrastructure of retinal pigment epithelial (RPE) mitochondria was carried out in chicks injected intravitreally with formoguanamine regularly (every 4 days) over the first 2 weeks or 4 weeks post-hatching. Formoguanamine is a triazine drug which induces hyperor-nithinemic symptoms in the chick eye similar to those seen in human gyrate atrophy.Results: A large population of irregularly shaped mitochondria was observed in the RPE of both peripheral and central retina. They showed extensive morphological changes. At 2 wk,the mitochondria appeared enlarged and abnormal in shape with vacuolisation, partial loss of their double membrane and reduced mitochon-drial cristae. By 4 wk, the mitochondria had assumed a rounder, almost circular profile,many with central holes,so-called ring mitochondria.Conclusi Purpose: To investigate the mechanism and sequence of formation of ring-shaped mitochondria in retinal pigment epithelial cells of a chick model of gyrate atrophy. Methods: Electron microscopic analysis of the ultrastructure of retinal pigment epithelial (RPE) mitochondria was carried out in chicks Formoguanamine is a triazine drug which induces hyperor-nithinemic symptoms in the chick eye similar to those seen in human gyrate atrophy. Results: A large population of irregularly shaped mitochondria was observed in the RPE of both peripheral and central retina. They showed extensive morphological changes. At 2 wk, the mitochondria was enlarged and abnormal in shape with vacuolisation, partial loss of their double membrane and reduced mitochon-drial cristae. By 4 wk, the mitochondria had assumed a rounder, almost circular profile, many with central holes, so-called ring mit ochondria.Conclusi