AbstractThe effect of ambient temperature and running activity on fetal outcome was studied in CD‐1 mice. Pregnant mice were allowed to be active in a running wheel at various ambient temperatures (26, 30, 32, 34, or 36°C) for 100 min a day. The dams were killed near term, and various maternal and fetal measures made. Mean deep body temperature (measured using radio‐telethermometers implanted in the abdomen) of pregnant dams was raised to 39.5°C while running at an ambient temperature of 36°C. Bred mice, pregnant or not, continued to increase their running activity, but pregnant mice exercised less from mid‐pregnancy on. Running up to 1 km/hr had no effect on maternal weight gain, number of implantations, number of live fetuses, fetal body weight, and fetal relative brain weight. However, increasing ambient temperature was effective in decreasing maternal weight gain and fetal body weight and increasing fetal relative brain weight. Even though body temperature can be increased significantly by increasing either running rate or ambient temperature, the body temperature increase caused by running appears to have no significant influence on the outcome of pregnancy. At levels in this study, body temperature per se does not appear to be the variable on which to predict fetal effects. This is because only body temperature raised by higher ambient temperature, and not body temperature raised by increased running, was shown to effect a feta