The decline of mammary epithelial cell (MEC) number during mammary gland involution in the cow is due to inhibition of proliferation and induction of apoptosis. Transforming growth factor-beta 1 (TGF-β1) belongs to a group of intramammary auto/paracrine inhibitors of bovine MEC growth and inducers of apoptosis. However, the mechanism responsible for the regulation of TGF-β1 expression in MEC is not known. The present study examined the effect of the hormones, growth hormone (GH), somatostatin (STS), 17-β oestradiol (E2), progesterone (P4), as well as the growth factors, insulin-like growth factor I (IGF-I) and epidermal growth factor (EGF), on TGF-β1 expression in the bovine MEC lines, BME-UV1 and MAC-T. The model of apoptosis in bovine mammary gland in vitro was applied by reduction of fetal bovine serum (FBS) (from 10% to 2% or 0·5% FBS) in the cell environment to show the relationship between TGF-β1 expression and apoptosis in bovine MEC. RT-PCR, Western blot and laser scanning cytometry (LSC) were used for analysis of TGF-β1 transcript and protein level as well as apoptosis and cell cycle in examined MEC. In this model of apoptosis, FBS deficiency (mimicking the naturally occurring decline in the access of bioactive compounds and nutrients at the end of lactation and dry period) was associated with increased TGF-β1 expression at the level of transcript and protein, induction of apoptosis and inhibition of cell cycle. Exogenous TGF-β1, IGF-I, EGF and GH inhibited FBS-deficiency-stimulated TGF-β1 expression. The suppressive effect of GH was reversed when cells were maintained longer in FBS-deficient medium. In general, STS, E2 and P4 increased TGF-β1 expression. However, this effect was dependent on hormone concentration and cell line. BME-UV1 cells were much more responsive to the peptides, GH, STS, IGF-I and EGF, whereas MAC-T cells were more responsive to the steroid sex hormones: E2 and P4.