The interaction between drugs which affect ON and OFF responses to light and ocular growth was investigated in 154 eyes of newly-hatched chicks raised with normal vision or monocular occlusion with regular intravitreal injections of APB (2-amino-4 phosphonobutyric acid) or PDA (cis 2,3-piperidine-dicarboxylic acid). The experimental results indicate that APB, at a dose sufficient to abolish the b-wave of the electroretinogram (but not to show extensive damage to the retinal neurons at the light microscopic level), caused a significant decrease in the axial growth of the eyes when compared with normal eyes. APB did not result in a significant difference in the growth rates of the occluded eyes compared with occluded controls. By contrast, injection of PDA into occluded eyes at a dose which reduced the response at light offset in the electroretinogram (and also affected the ON-response), caused a dramatic reduction in elongation of those eyes compared with occluded controls. Injection of PDA into eyes raised in a normal visual environment did not induce a significant difference from the growth rate of normally reared control eyes. These growth changes support the hypothesis that drugs which affect the physiological function of the retina, in particular, the strength of the ON and OFF responses, interact with the visual rearing environment to cause a consistent pattern of changes to eye growth and refractive error.