The study investigated the efficacy of fetal heart diameter for estimating gestational age by using transrectal ultrasonograph. For this purpose, two experiments (two stages of study) were performed on 36 fat-tailed sheep. In the first experiment, when the pregnancies had been detected 30 days after breeding (day 0), fetal heart, biparietal and trunk diameters were measured at 15-day intervals between days 40 and 100 of pregnancy to compare the estimated and actual gestational ages in 28 ewes. In the second experiment - a blind study - gestational ages were estimated at unknown ages of pregnancy by the operator in 8 ewes. There were positive correlations (p < 0.01) between the estimated and actual gestational ages determined by fetal heart, biparietal and body trunk diameters in stage I and stage II of the study, respectively (r = 0.99, r = 0.99; r = 0.99 and r = 0.89, r = 0.91, r = 0.97). In conclusion, transrectal ultrasonographic examination was reliable tool to estimate gestational age between days 40 and 100 of pregnancy, and estimatiing gestational age by fetal heart diameter was as reliable as that estimated by biparietal and trunk diameters.