['To study the ventilatory effects of closely monitored cycle ergometer conditioning (HR target, 145-150 beats x min(-1); 25 min/session; three sessions per week) during the second and third pregnancy trimesters (TMs) in healthy human pregnancy.', 'Subjects were 27 previously sedentary pregnant women (exercised group, EG). A sedentary control group (CG, N = 20) was also studied. Subjects were tested at rest and during upright cycle ergometry at three steady-state power outputs (approximately 20, 45, and 65 W) at the start of the second TM (ENTRY), at the end of the second and third TMs (post-training), and 3 months postpartum (PP, nonpregnant control).', 'Significant reductions in exercise HR were observed in the EG at each exercise level, confirming achievement of an aerobic conditioning effect. In both groups the ventilatory equivalent for oxygen (VE/O2) was significantly higher at all work rates during pregnancy compared with that during PP. This effect was caused by significantly higher exercise tidal volumes (VT) at each work rate in both groups during pregnancy versus PP. Respiratory sensitivity as reflected by the ventilatory equivalent for carbon dioxide (E/VECO2) and calculated arterial carbon dioxide tension (PaCO2) was significantly increased at all three work rates in late gestation versus PP and increased slightly with advancing gestational age in both groups. The main effects of physical conditioning were attenuation of increases in VE/VO2 observed at all exercise levels between the end of TM2 and TM3 and reductions in respiratory perception of effort (RPEr) between ENTRY and the end of TM3. Attenuation of increases in VE/VO2 during TM3 in the EG was attributed to the combined effects of enhanced fat versus carbohydrate utilization (resulting in reduced CO2 output and drive to ventilate) and attenuation of pregnancy-induced increases in dead space ventilation in late gestation.'] Does physical conditioning attenuate respiratory responses to steady-state exercise in late gestation?