The widely use of electrophysiological techniques as a non invasive method for evaluating the Central Nervous System (CNS) is well known. The electroretinogram (ERG) has proved to be useful in measuring retinal function, however adequate tracing have been obtained only recently. ERGs can now be done in newborn infants and toddlers. As part of study that evaluates the influence of diet in developing CNS, full field ERG were carried out in 21 preterm infants, with birth weight between 1000 and 1750 grs, who laked major illnes and were breast fed, at 36, 40 and 57 weeks (w) post conception. After 30 minutes of being in the dark, a luminous stimulus (flash like) was applied, using a 0.5 Hz frequency. A red filter (605 nm) was used to obtain an isolated response from cones and a blue filter (440 n) for rods, both with increasing intensity. The result obtained show that the rods threshold decreases significantly from-0.10 Ulog at 36 w. to-0.77 U log at 57 w. (p<0.05). The maximal retinal response (W16) amplitude (amp)increased from 33.48 mV at 36w., to 96.08 mV at 57 w.; the isolated cones response (BKD 16) amp from 14.51 mV at 36 w to 24.04 mV at 40 w., being both statistically significant (p < 0.05). The W16 implicit time (it) decreased from 81.60 mseg at 36 weeks to 68.16 mseg at 57 w., and it for BKD. 16 decreased from 51.82 mseg at 40 w. to 33.60 mseg at 57 w. We conclude that the difference in response observed in ERG's at different ages are in agreement with the process of retinal maturation and therefore it reflects the progression of the CNS maturation.