Milk yield can be affected by treatment with growth hormone (GH) and/or changes in milking frequency. Our study provides evidence that the mechanisms underlying these changes involved the modulation of mammary cell numbers in goats. Milking thrice daily (3X) compared with once daily (1X) for 23 days significantly affected milk yield, udder weight, the number of mammary epithelial cells and the level of the Bcl-2/Bax mRNA ratio, suggesting that 3X limits apoptosis when compared with 1X. GH treatment acted only on udder weight. These results demonstrate that milking frequency and GH act differently on epithelial cell number inside the alveoli.