Corpora lutea were collected from cows on Days 6, 8, 10, 12, 14, 16, 18 and 19 of the estrous cycle and early pregnancy (n=2/d) and were examined by light microscopy. Mean lutein cell diameter was significantly (P<0.05) greater in pregnant than in cyclic cows on Days 6, 8, 10, 12, 16, 18 and 19 (cyclic versus pregnant: Day 6: 13.9 +/- 0.22 vs 14.9 +/- 0.24; Day 8: 13.8 +/- 0.20 vs 15.4 +/- 0.2; Day 10: 14.8 +/- 0.24 vs 17.4 +/- 0.24; Day 12: 13.2 +/-0.25 vs 17.9 +/- 0.31; Day 16: 13.9 +/- 0.28 vs 16.5 +/- 0.31; Day 18: 13.0 +/- 0.22 vs 16.5 +/- 09.36, and Day 19: 15.0 +/- 0.23 vs 17.6 +/- 0.33 mum, respectively). The distribution of cell sizes was leptokurtotic throughout the estrous cycle and the first 10 d of pregnancy, but tended towards bimodality after Day 14 of pregnancy. The proportion of lutein cell cytoplasm occupied by vacuoles was lower in pregnant than in cyclic cows from the 12th day post estrus, but there was a marked (P<0.05) increase in vacuolation of cells from cows undergoing luteolysis. Stainable intercellular collagen was also less abundant in pregnant than cyclic cows from the 12th day post estrus. The higher rate of progesterone secretion of pregnant, compared with cyclic cows may be attributed to the greater numbers and greater contribution to luteal mass of large lutein cells in the corpus luteum of pregnancy.