This study used recently available, continuous noninvasive monitoring techniques to evaluate positional variations in pulse, blood pressure, and maternal oxygen saturation in 42 women undergoing fetal nonstress testing in the third trimester. Ten nonpregnant women were similarly evaluated with the automatic sphymomanometer and pulse oximeter. Six of 42 pregnant women (14.3%) developed the supine hypotensive syndrome (defined as a mean blood pressure decrease of 15 mmHg and a sustained increase in pulse of 20 beats per minute) when in the supine position. Nine of them (21.4%) met at least one of the criteria, but the majority (27 of 42, 64.3%) met neither criterion. None of the ten nonpregnant subjects had hypotension or tachycardia, although nine demonstrated blood pressure elevation after assuming the supine position. Significant oxygen desaturation did not occur in any patient, although three of six hypotensive patients had a transient 3-5% desaturation after supine rest. This study confirms that a significant percentage of patients in the third trimester are affected to some degree by supine hypotension. However, significant oxygen desaturation does not appear to occur.