Diagnoses of pregnancy were made on 110 Hereford cross Friesian and 69 blue grey (white shorthorn cross Galloway) cows between 92 and 202 days after last service using a real-time ultrasonic scanning instrument with a 3.5 MHz rectal transducer. Of the 174 cows which subsequently calved, one was wrongly diagnosed as non-pregnant. Of the five cows which did not subsequently calve two were diagnosed as pregnant and may in fact have been pregnant at the time of scanning. The overall level of accuracy of pregnancy diagnosis was 98.3 per cent. In further trials with 16 Hereford cross Friesian and 16 blue grey cows scanned at regular intervals between 20 and 140 days of gestation, pregnancy was diagnosed with confidence from 30 days, and relationships were established whereby gestational age could be estimated from measurements of certain uterine and fetal dimensions. Crown-rump length provided the most precise estimate of gestational age (residual sd +/- 4.5 days) and uterine diameter the least (+/- 12.6 days) with head length and the diameters of trunk, head and nose being intermediate (+/- 6.9 to 8.7 days).