Segments of basilar and middle cerebral arteries of eight human preterm and early postnatal infants have been examined using the resistance artery myograph technique for wire-mounted segments and the pressure perfusion arteriograph. Myographmounted segments spontaneously developed tone of varying duration and time course. Perfused segments showed maintained tone levels of approximately 40% of maximum possible constriction when the intraluminal pressure was 60 mm Hg. This level is not different from that found in adult human pial arteries of similar lumen diameter. Indomethacin (10-5M) either initiated tone increase or potentiated existing tone in the isometrically mounted segments. After washout of vasoconstrictors norepinephrine (10-6M) and angiotensin II (10-8M), indomethacin caused a pronounced, long lasting increase in basal tone. Spontaneous tone was reversed by acetylcholine (10-6M), isoproterenol(10-8to 10-5M), histamine (10-8to 10-5M), and papaverine (10-5M). Low levels of tone were increased and higher levels decreased by intraluminal flow. The pressure/diameter curves of these vessels were of similar shape as those of the equivalent size in the adult. It is concluded that intrinsic tone is a prominent feature of these large cerebral arteries, and it is modified by an endogenous indomethacin-sensitive process.Abbreviations: PSS,Physiologic saline solution;PG,prostaglandin