Abstract : The medium-term effects of permanent or random exposure to stray voltage applied to the water trough were evaluated on milk production and stress physiology in lactating dairy cows. Seventy-four Holstein cows were assigned during two 8-wk experimental periods to 1 of 3 treatments. The treatments were permanent exposure to voltage (PERM, 1.8 V, n=23) applied to the water trough, random exposure to voltage (RAND, 1.8 V, 36 h/wk, n=25), and no exposure to voltage (control, n=26). On the first day of voltage exposure, PERM cows had higher activity levels than control cows (9.8+/-2.70 vs. -2.3+/-2.74 14-s periods of movement/h). During the eighth week of exposure, RAND cows had higher activity levels than control cows (4.2+/-3.64 vs. -7.7+/-3.54 14-s periods of movement/h) and higher milk cortisol concentration than PERM cows (0.21+/-0.024 vs. 0.14+/-0.020 ng/mL). No differences were observed between treatments for cortisol response after an ACTH challenge during the seventh week of exposure. No effects of voltage exposure were observed on production traits and daily water intake. There was a transient decrease in milk yield on the second day of exposure in PERM cows (-1.4+/-0.74 kg) and on the third day of exposure in RAND cows (-3.5+/-1.03 kg) compared with control cows. In dairy cows, permanent or random exposure to stray voltage (1.8 V; 3.6 mA) could induce a transient stress response. Moreover, unpredictable voltage exposure could be considered a mild stressor, with slight modifications in stress physiology and activity but no impairment in production in the medium term