There is substantial evidence showing that the polymorphism of the goat αs1-casein (CSN1S1) gene has a major effect on milk protein, casein and fat content as well as on cheese yield. However, its influence on the synthesis rate of CSN1S1 has been less studied, with measurements only available in French breeds. In this article, we have measured milk CSN1S1 content in 89 Malagueña and 138 Murciano-Granadina goats with 305 and 460 phenotypic registers, respectively. In the Malagueña breed, average values of CSN1S1 content estimated for BB, BF, EE and FF genotypes were 6·94±0·38, 5·36±0·22, 4·58±0·13 and 3·98±0·27 g/l, respectively, being all significantly different (P<0·05). Conversely, in the Murciano-Granadina breed only the BB genotype (8·50±0·60 g/l) was significantly associated with increased levels of CSN1S1 (P<0·05), whereas BF (6·56±0·82 g/l), EE (6·39±0·60 g/l) and EF (6·91±0·76 g/l) genotypes displayed non-significant differences when compared with each other. Our results highlight the existence of breed-specific genetic and/or environmental factors modulating the impact of the CSN1S1 gene polymorphism on the synthesis rate of the corresponding protein.