The effect of nitrous oxide on cortical cerebral blood flow (CBF) was examined during a varying background anesthetic state in the New Zealand White rabbit. Seventy percent nitrous oxide resulted in significant and similar increases in CBF during anesthesia with both 0.5 MAC of halothane (44 +/- 14 to 63 +/- 17 ml.100 g-1.min-1) (mean +/- SD) and anesthesia with isoflurane (34 +/- 9 to 41 +/- 11 ml.100 g-1.min-1). During anesthesia with 1.0 MAC halothane or isoflurane, N2O also increased CBF, but the increments (halothane, 73 +/- 34 to 111 +/- 54 ml.100 g-1 min-1; isoflurane 34 +/- 13 to 69 +/- 34 ml.100 g-1.min-1) were significantly greater than those observed at 0.5 MAC. When 0.5 MAC halothane or isoflurane was supplemented with morphine (10 mg/kg followed by an infusion of 2 mg.kg-1.min-1), the CBF effect of N2O was not significantly different from that observed with 0.5 MAC alone. It was concluded that, in the rabbit, the effects of N2O on cortical CBF vary with the background anesthetic state and that the increase in CBF caused by N2O becomes greater as the end-tidal concentration of halothane or isoflurane increases from 0.5 to 1.0 MAC. Morphine, when added to 0.5 MAC of halothane or isoflurane, does not alter the effect of 70% N2O on cortical CBF.