Rat fetuses were extra-abdominaly exposed to 2 MHz continuous wave ultrasound, with 1.5 or 2.5W/c㎡ of spatial average (SATA) intensity, for 15 min at 7.5 to 11.5 days of pregnancy. Fetal anomalies were observed only in the groups exposed to 2.5 and 1.5W/c㎡ at 9.5 days of pregnancy but its occurrence was not statistically significant. Intrauterine fetal deaths were most prevalent in the group exposed to 2.5W/c㎡ at 9.5 days of pregnancy, followed by the group exposed to 2.5W/cm2 at 10.5 and 7.5 days of pregnancy. At three months after birth, the behavior (open field test), the learning ability (avoiding study and single straight pathway test) and the fertility of these exposed rats were compared with those in non-exposed rats. In the avoiding study, there was statistically less avoidability in the group exposed to 2.5W/c㎡ at 9.5 days of pregnancy than the non-exposed group. In the other tests, no significant differences could be found between the exposed group and non-exposed group. These findings indicated that rat fetuses at 9.5 days of pregnancy are most senstive to continuous wave ultrasound exposure followed by fetuses at 10.5 or 7.5 days of pregnancy.