Heat stress induces mammary apoptosis in vitro and dietary Zn alters mammary function, but their impact on mammary apoptosis and growth in vivo is unclear. During summer, 30 multiparous Holstein cows (days in milk = 99.7 d) were randomly assigned to 4 treatments arranged as a 2 × 2 factorial including 2 Zn sources mixed in the total mixed ration, 75 ppm Zn hydroxychloride (IZ) or 35 ppm Zn hydroxychloride + 40 ppm Zn-Met complex (ZMC), and 2 environments, cooling (CL) and not cooled (NC). The experiment was divided into baseline and environmental challenge (EC) phases, 84 d each. During baseline phase, all cows were cooled with fans and misters (average temperature-humidity index [THI] = 73) and fed respective dietary treatments, while during EC, NC cows were deprived of cooling (average THI = 78). Mammary biopsies were collected at −84, 7, and 56 d relative to EC to analyze gene expression related to apoptotic pathways [anti-apoptotic protein B-cell CLL/lymphoma 2 (Bcl2), and the pro-apoptotic proteins BCL2-associated X protein (Bax), Fas cell surface death receptor (Fas), and caspase-3] by qRT-PCR, and apoptosis and cell proliferation by immunohistochemistry. Data were analyzed using PROC MIXED and GLIMMIX of SAS 9.4. Expression of Bcl2 and Bax tended (P < 0.11) to be higher for NC compared with CL. The NC cows had higher expression of Fas and tended to have higher gene expression of caspase 3 at d 7 (CL × d, P < 0.03) than CL. Relative to IZ, ZMC tended to have higher mRNA expression of Fas at d 7 but tended to have lower Bax expression at d 56 (Zn × d, P < 06). Expression of Fas of IZCL cows was lower than IZNC and ZMCCL (Zn × CL, P = 0.02). Compared with IZ, ZMC cows tended to have lower (P = 0.09) total mammary cell proliferation and higher (P < 0.01) apoptosis. Relative to CL, NC cows had higher mammary cell proliferation rate at d 56 (CL × d, P = 0.07). In conclusion, cooling reduced gene expression involved in apoptotic pathways and decreased cell proliferation. Relative to IZ, feeding ZMC lowered mammary cell proliferation but increased apoptosis; however, the cellular mechanism is unclear.