We have developed a lightweight plastic goggle with rigid contact lens inserts that can be applied to the eyes of newly hatched chicks to explore the range and accuracy of the developmental mechanism that responds to retinal defocus. Convex and concave lenses of 5, 10, 15, 20 and +30 D were applied to one eye on the day of hatching. The chick eye responds accurately to defocus between -10 and +15 D, although hyperopia develops more rapidly than myopia. Beyond this range there is first a levelling off of the response and then a decrease. The resulting refractive errors are caused mainly by increases and decreases in axial length, although high levels of hyperopia are associated with corneal flattening. If +/- 10 D defocusing lenses are applied nine days after hatching the resulting myopia and hyperopia are equal to about 80% of the inducing power. After one week of inducing myopia and hyperopia with +/- 10 D lenses, the inducing lenses were reversed. In this case, the refractive error did not reach the power of the second lens after another week of wear. Instead, astigmatism in varying amounts (0-12 D) was produced, being greater when reversal was from plus to minus. Finally, astigmatism can also be produced by applying 9 D toric inducing lenses on the day of hatching. The astigmatism produced varies from 2 to 6 D, and the most myopic meridian coincides with the power meridian of the inducing lens. This astigmatism appears to be primarily due to corneal toricity.(ABSTRACT TRUNCATED AT 250 WORDS)