Postnatal development of blood pressure, heart rate and their regulation by arterial baroreceptor reflex in mice was examined. We first confirmed that simultaneous recordings of pulsatile blood pressure by the "servo null" method and the conventional catheter method gave almost identical tracings in halothane-anesthetized adult mice. We then measured blood pressure by servo null method together with electrocardiograph in mice of various ages from newborn to adult. Mean blood pressure increased progressively with age from 19 + 2 mm Hg in P0 newborn to 74+/-1 in adult mice, while heart rate initially increased from 365+/-12 bpm in newborn to 441+/-15 in infant (7 days old), and then decreased to 337+/-15 in adult mice. Between 1 and 2 weeks of age, gain of arterial baroreceptor reflex abruptly increased from a newborn value of 0.3 to a near adult value of 1.1 ms/mm Hg. On the other hand, sensitivity to anesthesia did not differ except for P1 and P2 newborns. We conclude that pulsatile blood pressure can be accurately measured by the servo null method even in the newborn mice and that baroreflex heart rate control mature at around 2 weeks after birth in the mice.