The effects of fentanyl or cardiovascular function, regional distribution of cardiac output, and the dose required for producing anesthesia were studied in ten previously catheterized, newborn lambs. In addition, the effects of fentanyl on cerebral blood flow and oxygen utilization were examined. Fentanyl in cumulative doses as high as 4.4 mg/kg (average plasma levels of 646 +/- 95 ng/ml, maximum plasma level of 862 ng/ml) did not reliably produce anesthesia as assessed by tail and foot clamping, although it did cause profound respiratory depression. With normocapnia maintained by mechanical ventilation, fentanyl did not alter cerebral oxygen delivery or consumption, and the two remained coupled. Fentanyl did not affect cardiac output, heart rate, or mean arterial blood pressure at the highest dose level, nor did it reduce blood flow to specific organ beds, other than the kidney. Thus, the hemodynamic stability seen with fentanyl in the lamb does not occur at the expense of reduced blood flow to organs, such as the gastrointestinal tract or brain, that are particularly vulnerable in the neonate.