The purpose of this study was to determine if pre- and post-hatching patterns of insulin-like growth factor-I (IGF-I) RNA and polypeptide expression are related to growth characteristics of three genetic stocks of chicken. Significant differences in serum IGF-I concentrations were found for different genetic stocks $lt;Athens-Canadian Randombred (ACRB), Single Comb White Leghorn (SCWL) and Peterson Arbor Acres (PXAA) commercial broiler$gt; of chicken at 6 and 8 days post-hatching. These results reflect differences in growth pattern among the three generic stocks of chicken. Therefore the growth characteristics during the first and the second week of the chicken might be critical for overall growth. IGF-I RNA expression in muscle and liver was determined at each stage showing an overall increase with age; however, no apparent relationships between IGF-I RNA expression and growth differences among genetic stocks were found. Levels of IGF-I RNA in liver showed great variability during sampling period.