Follicular and luteal changes during the first half of pregnancy were studied using 47 Yorkshire (Y), 52 Landrace (Ld) and 44 Lacombe (Lc) sows examined at 23, 42 and 63 days of pregnancy. Follicular fluid weight was greater in Lc than in Y (20%) and Ld (8%) sows. The mean follicular weight for all breeds increased by 22.6% (P < 0.05) from day 23 to day 42 of pregnancy. Yorkshire sows had significantly more small (1–2 mm) follicles than Lc and Ld sows. The average number of small follicles for all breeds was 117 at day 23 and remained unchanged at day 42, but was 20.5% higher (P < 0.01) at day 63. The number of medium (3–5 mm) follicles was similar in the three breeds, whereas that of large follicles (6–10 mm) was greater in Lc than in Y and Ld sows. The number of medium and large follicles increased with the advance in pregnancy. The ovulation rate as measured by the number of corpora lutea was 15.1 in Ld and 14.1 in Y and Lc. Luteal tissue weight was less in Y than in Ld and Lc sows. The weight of luteal tissue increased with advancing pregnancy.