Retinoic acid (RA) is an important player in the developing systems. The present study aimed to investigate the effect of exogenous RA on pattern formation of thr developing chick embryo. Fertilized eggs of Gallus domesticus were injected with a single dose of 0.5 µg, 1 µg or 2 µg of RA dissolved in 1 µl of dimethylsulphoxide (DMSO). Injections were carried out after three days of incubation. RA treatment induced teratogenic effects during the morphogenesis of the developing chick embryos. These effects included eye abnormalities such as lens dysgenesis, retinal folds and chorioretinal coloboma. The effects also included several abnormalities and hypoformations in the trunk and tail regions, fore- and hind-limb abnormalities, and anteriorly shifted hind limbs. Also, dwarfism and disappearance of limb parts were observed. The explanation of the RA effect based on the disturbance of normal pattern of expression of genes concerned with the process of morphogenesis, where the excess of the exogenous RA cause overexpression of some genes resulted in several malformations and disturbance of pattern formation in the developing embryo. Also, RA might have induced cell death in some parts of the limbs leading to disappearance of toes in some cases and syndactyly in other cases.