Growth hormone, a polypeptide encoded by the GH gene, is a member of the somatotropin/prolactin family of hormones, which plays an important role on milk production and metabolism. In bovine, a single nucleotide polymorphism (SNP) in exon 5 changes leucine to valine (CTG to GTG) in the mature GH molecule. The purpose of this study was to evaluate, in Modicana and Cinisara cows, the effects of genetic polymorphism at GH locus and its interaction with feeding system on milk traits. A total of 97 individual Modicana blood samples were collected, 65 from a semi extensive and 32 from an extensive farm. A total of 58 Cinisara individual milk samples were collected, 32 from 7 extensive farms and 26 from 6 semi-extensive farms. In the extensive farms the cows were fed exclusively with pasture; in the semi-extensive farms cows were fed with pasture, hay and concentrate. DNA was extracted from Modicana white blood cells and from Cinisara milk somatic cells. Bovine Growth Hormone (bGH) was analyzed at the 5th exon according to Komisarek et al (2011). In Modicana farms, monthly, individual milk yield was recorded and individual milk samples collected. In Cinisara farms milk production was recorded once, the day of samples collection. Fat and protein were determined (Combi-foss 6000, Foss Electric, Hillerød, Denmark). In Modicana, milk yield, fat and protein were analysed using the GLM procedure for repeated measures of SPSS (SPSS for Windows, SPSS Inc., Chicago IL, USA). In Cinisara, milk yield, fat and protein were analysed using the univariate GLM procedure of SPSS. The analyses included main effects of GH genotype, feeding system and the interaction genotype × feeding system. Three genotypes were found:: LL (236, 132, 36bp), LV (236, 185, 132, 51, 36 bp) and VV (185, 132, 51, 36 bp). Genotype frequencies, respectively in Modicana and Cinisara breeds were: LL, 0.61 and 0.34; LV, 0.37 and 0.57; VV, 0.02 and 0.09. Milk production and gross composition were not influenzed by GH genotype (respectively in Modicana and Cinisara: milk yield, LL 9.06 and 12.3, LV 9.24 and 11.4, VV 7.61 and 8.33; fat, LL 4.08 and 3.32, LV 3.91 and 3.13, VV 4.04 and 3.62; protein, LL 3.67 and 3.51, LV 3.66 and 3.56, VV 3.64 and 3.71). These results are in line with different authors. The higher energy level of the diets offered in the semi extensive farms did not interfere with GH genotype effect on milk traits: no significant interaction was found between genotype and feeding system.