A study was conducted to evaluate the effect of 2 cooling systems on barn temperature, core body temperature (CBT), respiration rate (RR), rear udder temperature, and lying time in lactating Holstein dairy cows. A switchback design was used where cows were moved between barns for 6 d, therefore exposing treatment groups to each barn for 3 d. Twenty lactating Holstein dairy cows were randomly assigned to 1 of 2 treatment groups: CONV refers to cows housed in a conventional, open-sidewall freestall barn using feedline soakers and fans located over the feedline and stalls as its main source of cow cooling, and TUN + EVAP, which refers to cows housed in a tunnel-ventilated freestall barn using an evaporative cooling system provided by VES Environmental Solutions (Chippewa Falls, WI). The cooling system in the tunnel-ventilated barn (TUN + EVAP) was effective at reducing barn temperature and temperature humidity index (THI), while relative humidity (RH) was increased in comparison CONV. Lower THI of the cow environment for TUN + EVAP failed to result in treatment differences for CBT, however, with CONV and TUN + EVAP having similar CBT of 38.6 ± 0.04°C (P = 0.79). TUN + EVAP cows had reduced RR (P < 0.01) compared with CONV (52.0 vs 57.9 ± 2.2, respectively) and this difference was greater during the afternoon h (1600 h) with average RR of 55.4 and 63.0 ± 2.6 for TUN + EVAP and CONV, respectively (P < 0.01). Similar results were found for rear udder temperatures where TUN + EVAP cows had reduced rear udder temperatures overall (33.2 vs 34.5 ± 0.3°C; P < 0.01) and during the afternoon period (34.0 vs 34.9 ± 0.4°C; P < 0.01) compared with CONV. Cows housed in the TUN + EVAP barn had increased lying time by 1 h/d (P < 0.01) compared with CONV (11.8 vs 10.8 ± 0.3 h/d for TUN + EVAP and CONV, respectively). Overall, the evaporative cooling system was effective in reducing the barn THI leading to reduced RR and rear udder temperature and increased daily lying time. No treatment differences were detected for CBT, however, likely a result of the cooler ambient conditions under which the study took place.