Objective  To investigate the characteristics of the dark-adapted electroretinography (ERG) a-wave during development in normal rats. Methods  In this experimental study， the dark-adapted ERG of eight groups of Rcs-rdy+-p+rats at different ages (21， 25， 32， 35， 37， 46， 60 and 90 d)were recorded by a series of flashes (0.03， 0.95， 3 and 9.5 cd•s•m-2) using a RETI-scan system and gold-foil ring form corneal recording electrode and a stainless steel needle electrode custom made in the lab. Data were exported to the computer and processing by a Matlab 7.4 software， then were modeled with the solver model of Excel and two parameters of the a-wave were analyzed： the saturation a-wave amplitude (Rmp3) and sensitivity (S). Results  The minimum Rmp3 appeared at 25 days old group. When the intensity of light was 0.03 cd•s•m-2， the maximum of Rmp3 appeared at 60 days old group， under other intensity of light， it appeared at 46 days old group. It showed that there was a linear relationship between the sensitivity of the 8 groups and intensity of the light (S=ks•I+bs). Excluding the 60 days old group， the effects of age was mainly on the constant—bs but not the slope of the curve—ks， ks was equal to 0.09. Conclusion  The Rmp3 and sensitivity of a-wave of the ERG can be affected by age with vary degrees.