We studied the value of Pourcelot's index of resistance in the anterior cerebral arteries (RI-ACA) and basilar artery (RI-BA) in 69 measurements of term infants on days 1, 5, 10, 15, 20, 30 and 60. The mean value of RI-ACA (0.723 +/- 0.038, +/- SD) was significantly lower than that of RI-BA (0.750 +/- 0.041) (p less than 0.001). RI-ACA values were higher than RI-BAs in only five measurements (7.2%). The values of RI-ACA and RI-BA decreased from the first day to the fifth day of life. They increased continuously and showed their highest values on the day 15. After the 15th day, they decreased mildly again. The values of RI ratio (= RI-ACA/RI-BA) were stable (0.96-0.97).