The association of single nucleotide polymorphisms (SNPs) within genes of the prolactin (PRL), growth hormone (GH) and insulin-like growth factor-I (IGF1) pathways with 305-day milk yield (MY305) were evaluated in heat-stressed Holstein cows. Ambient temperature and relative humidity were used to calculate a temperature-humidity index (THI), which revealed that heat stress conditions existed in the Yaqui Valley, Sonora, México from May through October of 2011 and 2012. Cows (n = 573) were genotyped for 179 tag SNPs within 43 candidate genes in the PRL and GH/IGF1 pathways. Seven SNPs within 7 genes (AVPR1A, Furin, IGFBP5, IGFBP6, PMCH, PRLR and STAT5B) were found to be associated with MY305 (P<0.05); therefore, their effects were used to estimate a molecular breeding value (MBV). The correlation between the MBV and MY305 was 0.21 (P<0.001) and the adjusted coefficient of determination (R2) was 4%, whereas correlation between MBV and the estimated breeding value for MY305 was 0.25 (P<0.001) with a R2 of 6%. Heritability estimate for MY305 was 0.41 ± 0.12. The MBV estimated was positive, but weakly associated with MY305. The small amount of additive and phenotypic variation explained by the MBV was most likely due to the few number of SNPs and the complexity of the trait, particularly under extreme weather conditions. In conclusion, SNPs within the PRL and GH/IGF1 pathways were associated with MY305; however, the MBV estimated with these SNPs was not yet suitable to use in genetic selection procedures due to the small amount of variation explained for MY305.