BACKGROUND For vendor-derived mice, an acclimation period of 1 week is usually recommended before blood pressure measurements are started. However, we observed hypertension in wild-type vendor-derived mice 1 week after shipping. METHODS The index group (n = 12, BALB/c, age 3 months, weight 26-28 g) was shipped overnight (by truck, duration 13 h).Tail-cuff systolic blood pressures (SBPs) of the index group were compared to two control groups (n = 6/group), one acclimated for 3 weeks after shipping, and one derived from an in-house colony. RESULTS One week after shipping, SBP in the index group was 141 +/- 3 mm Hg. Because this was much higher than reported previously for this strain, acclimation was prolonged. Six weeks after shipping, SBP had fallen to 124 3 mm Hg (P < 0.005). During this time, heart rate also fell from 721 +/- 15 to 665 +/- 13 bpm (P < 0.01). SBP in the two control groups was also lower than in the index group 1 week after shipping, including the group acclimated for 3 weeks (129 +/- 3 vs. 141 +/- 3 mm Hg, P < 0.05) and the in-house mice (124 +/- 3 vs. 141 +/- 3 mm Hg, P < 0.005). CONCLUSIONS Vendor-derived mice are hypertensive 1 week after shipping, become normotensive after 3 weeks, but do not return to levels of in-house mice until after 6 weeks. Acclimation periods of at least 3 weeks are required when measuring blood pressure in mice.