The nature of cumulus--oocyte complexes was examined in PMSG (group 1, n = 18) and FSH (group 2, n = 30) stimulated heifers. Laparoscopy was performed 65 h after cloprostenol application. The number of follicles was 13.67 +/- 0.75 and 12.67 +/- 0.81 (P greater than 0.05) in group 1 and 2, respectively (Tab. I). The recovery rate of oocytes was 56% in the first and 67% in the second group (Tab. I). The cumulus oophorus was divided into three groups: compact, expanded and partial (Tab. II). Most oocytes (65 and 75% in the first and second group, respectively) exhibited an expanded cumulus (P greater than 0.05). In the first and second group 11 and 26% (P less than 0.01) of oocytes with the extruded first polar body were aspirated (Tab. III). As judged from the pool of visible follicles, the superovulation response to stimulatory treatment and recovery rate of oocytes in the present experiment were not different from the results published earlier. The degree of the cumulus oophorus expansion is an indicator for the evaluation of cumulus--oocyte complexes. After the preovulatory LH peak the disintegration of cumulus oophorus proceeds from glycosaminoglycan accumulation. In our experiment this effect resulted in a significantly higher number of oocytes with expanded cumulus in both treatments. The enlargement of perivitelline space is related to a subsequent release of the first polar body in the preovulatory period. It can be seen from our results that after FSH treatment it is possible to reach the high number of oocytes with the extruded polar body.(ABSTRACT TRUNCATED AT 250 WORDS)