Programmed cell death, apoptosis, is a mechanism to maintain tissue homeostasis. Although the apoptotic process in rodent mammary tissues has been known to occur at the onset of involution, little is known about programmed cell death in the bovine tissues. Therefore, the purpose of this study was to investigate the molecular and cellular basis of apoptotic process in bovine mammary cells. Mammary tissues were obtained at different lactational and involurional stages. By apoptosis in situ endlabeling assay, apoptotic cells were found around the acinar celt lining in regressing bovine mammary tissues. The apoptosis-related genes bel-2 and bax were detected throughout involution by Northern blotting assay. The level of bax mRNA was dominantly expressed during involution. On the other hand, the bel-2 RNA transcripts were constantly expressed by 14 of post-lactation and declined thereafter. The expression of the testosterone-repressed prostate message-2 (TRPM-2) RNA transcripts, a marker for tissue remodeling, was increased as involution progressed. TNF a, were induced the DNA fragmentation and enhanced the expression of bax mRNA. In addition, milk protein secretion and amino acid uptake were decreased in mammary acinar culture treated with TNF $alpha$. These results indicate that bovine mammary cells undergo apoptotic process after the cessation of milking and that TNF $alpha$ may trigger apoptosis in lactating bovine mammary acini.