Four points on the basal body temperature (B.B.T.) curve was correlated with the estimated time of ovulation, as determined by serial ultrasound in 50 induced menstrual cycles from 22 subjects. The time of ovulation was estimated by measuring the maximal diameter of follicles and observing the morphologic changes within the ovary from follicle to corpus luteum. The results were as following; 1. The diameter of the follicle measured at the day before disappearance was 21.1 mm on an average (S.D.: 2.14). The average follicular growth for 4 days before ovulation was measured at a rate of 2.8 mm/day, and rapid growth of follicle was observed 3.1 mm/day at the day before. 2. The changes associated with rupture of the follicles were the followings, in order of frequency; decrease in size(94%), disappearance of follicles(64%), fluid in the Cul-de-Sac(26%) and increased internal echoes(16%). 3. Only 20 of 50 cycles, exhibited a BBT dip and correlated with the estimated time of ovulation by ultrasound in 2 of which cases(10%). BBT nadir, 30 of 50 cycles, correlated in 5(16.7%). The first day of hyperthermic plateau(FDHP) and BBT coverline was exhibited in all cycles, correlated in 41(82%) and 35(70%) cases. 4. The relationship between the diameter of dominant dominant follicle, measured by ultrasound, and the basal body temperature curve were as following. During cycles in which dip was observed on the BBT curve, the follicular diameter were 10.5${pm}$2.12 mm on 4 days prior to the point (D-4), and 12.5${pm}$2.12 mm (D-3), 15.5${pm$2.12 mm (D-2), 17.0${pm}$1.41 mm (D-1) and 21.5${pm}$2.12 mm just prior to the dip (D-0). In the nadir; 9.6${pm}$1.67 mm (N-4), 12.8${pm}$1.79 mm (N-3), 16.2${pm}$1.92 mm (N-2), 18.2${pm}$2.17 mm (N-1) and 21.4${pm}$2.61 mm (N-0). In the First day of Hyperthemic Plateau (FDHP); 9.8${pm}$1.36 mm (F-4), 12.4${pm}$1.41 mm (F-3),15.1${pm}$1.57 mm (F-2), 18.1${pm}$1.67 mm (F-1) and 21.2${pm}$2.25 mm (F-0). In the BBT coverline endopint; 9.9${pm}$.39 mm (C-4), 12.5 ${pm}$1.44 mm (C-3), 15.2${pm}$1.64 mm (C-2), 18.0 ${pm}$1.69 mm (C-1), and 21.2${pm}$2.31 mm (C-0). 5. The relationship between the ultrasonographic signs of ovulation and the basal body temperature curve were as following. The BBT dip correlated with the ovulation in 2 cases, which revealed decrease in follicular diameter (100%), fluid pattem in the Cul-de-Sac (1 case, 50%) and complete disappearance of follicle (1 case, 50%). In the nadir (5 cases); the ultrasonographic signs of ovulation were decrease in follicular diameter (5 cases, 100%), fluid pattern in the Cul-de-Sac (1 case, 20%) and complete disappearance of follicle (3 cases, 60%). In the First day of Hyperthermic Plateau (41 cases); decrease in follicular diameter (40 cases, 97.6%), fluid pattern in the Cul-de-Sac (11 cases, 26.8%), appearance of internal echo and thickening of the wall (6 cases, 14.6%) and com plete disappearance of follicle (28 cases, 68.3%). In the BBT coverline endpoint (35 cases); decrease in follicular diameter (33 cases, 94.3%), fluid pattern in the Cul-de Sac (9 cases, 25.7%), appearance of internal echo and thickening of the wall (5 cases 14.3%) and complete disappearance of follicle (20 cases, 57.1%).