Regular injections of the gliotoxins D-α-aminoadipic acid and L-α-aminoadipic acid were made into the vitreous chamber of the eyes of newly-hatched chicks reared either in a normal visual environment or under conditions of monocular occlusion. Striking differences in the growth rates of the eyes from the different groups were observed. Injection of D-α-aminoadipic acid, which causes the Müller glial cells to swell and diminishes the retinal OFF-response, resulted in a marked increase in the rate of axial growth of the eye compared with normal eyes. However, injection of D-α-aminoadipic acid into occluded eyes caused a lesser growth rate than was observed in occluded control eyes. By contrast, injection of L-α-aminoadipic acid, which also causes severe glial swelling and abolishes the retinal ON-response, caused reduced eye growth in non-occluded eyes. However, injection of L-α-aminoadipic acid into occluded eyes caused eye growth in excess of that recorded in the occluded controls. We concluded that the different growth rates observed is more likely a result of the disruption of the neural ON and OFF mechanisms than of the indisposition of the Müller cells.