Retinopathy of prematurity is a disorder of abnormal retinal vascular proliferation, and one hypothesis for its pathogenesis involves abnormal activity of angiogenic growth factors in the retina. One of these, acidic fibroblast growth factor, is found primarily in retina and brain tissues. Its mitogenic effect is greatly potentiated in vitro by heparin. Because retinopathy of prematurity occurs most often in premature infants who receive the greatest amount of heparin, we tested the hypothesis that heparin may adversely affect the retinopathy observed in kittens after hyperoxic (80% oxygen) exposure. Seventeen litters of kittens were randomly assigned to receive either saline or heparin s.c. injections from d 2 through recovery to 28 d of age; 65 h of high oxygen exposure was started on d 3 to induce a standard retinal injury in our model. There were no differences in the degree of retinopathy between the heparin-treated group [severity score 5.9 +/- 2.2 (mean +/- SD)], and the saline-treated group (severity score 7.1 +/- 1.7, p greater than 0.20, 80% power to detect a 2-point difference in score at alpha = 0.05). These findings do not support a concern that clinical doses of heparin potentiate retinopathy of prematurity.