To determine the limits of aortic blood pressure in infant lambs for autoregulation of global and regional brain blood flow, we studied 10 unsedated lambs during hypotension and 10 unsedated lambs during hypertension. In lambs 6 to 13 d old, we produced graded changes in aortic blood pressure by inflating a balloon occluder placed around either the inferior vena cava or the descending aorta. Using radiolabeled microspheres, we measured global and regional brain blood flow at the baseline, and then with each graded change in aortic blood pressure. In an additional step, we administered atropine to determine if its antimuscarinic properties alter the fall in brain blood flow with severe hypotension, or alter the rise in brain blood flow with severe hypertension. We concluded that in the unsedated infant lamb, global brain blood flow remains stable between mean aortic blood pressures of 6.0 to 10.0 kPa (45 to 82 torr), a range from approximately 38% below to 12% above normal mean aortic blood pressure. We noted that this autoregulatory range is essentially unchanged from that described for the fetal lamb at 80% of term gestation--even though the mean aortic blood pressure rises during this period of maturation by more than 2.7 kPa (20 torr). We found that the lower limit of autoregulation varies among the different brain regions and is lowest in the thalamus, pons, and medulla. We saw little variation of the upper limit among the brain regions. Finally, we determined that atropine does not alter brain blood flow during severe hypotension or severe hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)