The aim of this study was to investigate the role of vasoactive intestinal peptide (VIP) in the development of form deprivation myopia (FDM) in chicks. Myopia was induced monocularly by lid suture on day 6 after hatching, VIP injections were given by intravitreally (0.5 ng/10 mu l) or subconjunctivally (25 ng/50 mu l). Injections were repeated every 24 hour for a period of 8 days. Refractive development was monitored by streak retinoscopy and axial eye length was recorded by A-scan ultrasound measurements, In subconjunctival VIP injected chicks produced mean - 1.4+/-0.2 diopter of experimental myopia. In intravitreal VIP injected chicks had mean +3.5+/-0.7 diopter of refractive error. In control group monocularly deprived chicks produced mean -12.2+/-4.1 diopter of experimental myopia (p<0.001). This protective effect of VIP in experimentally induced myopia was associated with marked reduction in the relative axial elongation of the deprived eye when compared to phosphate buffer saline injected chicks or control monocularly deprived chicks (p<0.001). These results suggest that subconjunctival VIP suppresses, intravitreal VIP blocks I;DM in chicks.