Technical Abstract: Induced ovulation of small dominant follicles (sdf, <12 mm; CO-Synch protocol) in postpartum beef cows resulted in formation of CL that exhibited a delayed rise in progesterone (P4; P < 0.05) compared to CL that formed from large dominant follicles (ldf, >12 mm). The objective was to characterize P4 concentrations (0-60 d post AI) among GnRH-induced or spontaneously ovulated sdf (< or =12 mm) or ldf (> or =13mm) to determine whether P4 secretion by CL formed from GnRH-induced sdf remains lower during early gestation. Postpartum beef cows were induced to ovulate 48 h after PGF2a (CO-Synch) or undergo spontaneous estrus and ovulation. Follicle size was measured at AI in cows induced to ovulate or 12 h after onset of estrus for cows that ovulated spontaneously. Cows were classified into one of three groups: 1) sdf (< or =12 mm)- GnRH-induced ovulation (SF-I; n=9); 2) ldf (> or = 13 mm)-GnRH-induced ovulation (LF-I; n=38); or 3) ldf (> or =13 mm)-spontaneous estrus and ovulation (LF-S; n=26). Blood samples were collected every other day for 60 d beginning at AI (d 0). The rate of P4 secretion was increased (P=0.06) in pregnant (d 2-12) compared to nonpregnant cows. Although the rate of increase in P4 from d 2-12 was higher (P=0.01) in the LF-I compared to the SF-I groups, there was no difference (P=0.94) among groups in P4 from d 14-60 in pregnant cows. Pregnant cows in the LF-S group, however, had higher rate of increase in P4 from d14-60 compared to pregnant cows induced to ovulate. Follicle size at AI influenced the rate of P4 increase in cows that failed to conceive (P=0.007), but not among cows that became pregnant (P=0.32) to AI. The regression of follicle diameter at AI on serum P4 on d 6 was significant (P=0.002) and linear in cows induced to ovulate, but not among cows that ovulated spontaneously. In summary, P4 secretion following GnRH-induced ovulation of small dominant follicles was decreased from d 2-12 compared to large dominant follicles, but similar among pregnant cows from d 14-60 post AI (d 0).