There is a growing need to define the physiologic ‘steady state’ in pregnant rhesus monkeys because of the increased demand for this experimental model in investigation of maternalfetal relationships. As a first step toward this end maternal and fetal arterial BP, pulse, pH and blood gases were studied in the normal rhesus monkey, both awake and following pentobarbital surgical anesthesia. From our results we must deduce that the effects of the requisite experimental procedures cannot be discounted until at least 8 h have elapsed.