Abstract. The aim of our studies was to determine, whether difference in GH alleles affect growth Performance in cows and whether these effects may be mediated by IGF-I. The presence of L (leucine-rich) and V (valine-rich) alleles of GH, plasma IGF-I level and changes in body mass were analyzed in 84 bulls of Simmental (meat) and Holstein (milk) breeds at 90 d and 180 d of age using PCR, agarose gel electrophoresis and IRMA The agedependent increase in body mass, daily gain and plasma IGF-I concentration was observed in Simmental bulls. In Holstein bulls the age-dependent rise in plasma IGF-I was also found, whilst Holstein bulls had significantly lover IGF-I level, than Simmental bulls. The proportion of LL, VV and LV genotypes of GH in the randomly selected herd of Simmental bulls was 0.25:0.20:0.55. Animals of VV genotype had lower body mass, daily gain and plasma IGF-I level, than bulls of LL and LV genotypes, whilst no differences in these indexes between LL and LV genotypes were found. The observed association of growth rate, GH genotype and IGF-I level, suggest that GH genotype may affect meat Performance via IGF-I secretion.