The effect of ambient temperature T(sub a) and running wheel activity (RWA) on fetal outcome was studied in CD-1 mice. Pregnant mice were allowed to be active in a running wheel at various T(sub a)'s (26, 30, 32, 34 or 36C) for 100 mins a day. The dams were killed near term, and various maternal and fetal measurements made. Mean deep body temperature T(sub b) in pregnant dams while running (measured using radio-telethermometers implanted in the abdomen) was raised to 39.5 C at T(sub a)=36. Bred mice continued to increase their RWA, pregnant or not, but pregnant mice exercise less from mid-pregnancy on. RWA up to 1 km/hr had no effect on maternal weight gain, litter size, number of live fetuses, fetal body weight and fetal relative brain weight. However, increasing T(sub a) was effective in decreasing maternal weight gain and fetal body weight, and increasing fetal relative brain weight. Even though T(sub b) can be increased significantly by either RWA or T(sub a), the increase caused by RWA appears to have no significant influence on the outcome of pregnancy. At levels in this study, T(sub b) per se does not appear to be the variable on which to predict fetal effects. This is because only T(sub b) from T(sub a), and not T(sub b) from RWA, could be shown to effect a fetal change. (Copyright (c) 1990 Wiley-Liss, Inc.).