The effects of an aerobic walking program, from 22 to 30 wk gestation, on aerobic fitness, vitamin B-6 status, and birth outcome was studied in 28 healthy, pregnant women aged 21-36 yr receiving vitamin-mineral supplements. Aerobic fitness was assessed by heart rate (HR) and relative oxygen consumption (VO₂, ml/kg/min) during 2 submaximal treadmill walking tests (22 and 30 wk). HR responses were significantly lower for walking subjects (W; n=18) compared to nonwalking (NW; n=10) at 2 and 4 min and near significance at 6 min of the 30 wk treadmill test. HR increased significantly at 2 min for NW from 22 to 30 wk. Oxygen consumption significantly decreased for W from 22 to 30 wk at 2, 4, and 6 min of exercise but remained unchanged for NW. Mean vitamin B-6 intake, minus the 10 mg supplement, was at least two-thirds the 1980 Recommended Dietary Allowance. Values for plasma total vitamin B-6 assessed microbiologically were in the low-normal range for only 3 subjects (1 W at 22 wk and 2 W at 30 wk). Values for plasma pyridoxal phosphate levels assessed radioenzymatically were in the low-normal range for only 2 subjects (1 W at 22 wk and 1 W at 30 wk). Birth weight, Apgar scores, and labor duration were similar for both groups. Participation in a walking program by pregnant women taking vitamin-mineral supplements slightly improved aerobic fitness without affecting vitamin B-6 status (probably due to the supplement usage) or birth outcome. / M.S.