The aim of the present study was to determine the degree of development and structural status of perfusion-fixed day-7 corpora lutea (CL) (using qualitative and quantitative histology at light and electron microscopy levels) in FSH-induced superovulated (SO) and untreated heifers. Blood samples were collected daily to monitor the plasma progesterone levels during the FSH treatment up to slaughter 7 days after oestrus. Meanwhile the ovarian activity was followed by ultrasonography and rectal palpation. At slaughter, the ovaries were fixed by vascular perfusion to avoid distortion of the structures and conventionally processed for electron microscopy. The volume density of the luteal tissue was calculated on the examined sections by point-counting. The histology of the corpora lutea, exception made of a higher incidence of degenerated luteal and endothelial cells in the superovulated animals, did not differ from that of the untreated animals, confirming their normal development. The mean weight and volume of the CL in SO animals was 50% smaller than that of the untreated heifers. The progesterone concentration (nmol/l) at day 7 was significantly positively correlated with the number of CLs/heifer (r = 0.93, P less than 0.01), with the weight of Cls/heifer (r = 0.97, P less than 0.005) and with the volume of CLs/heifer (r = 0.97, P less than 0.005). The results indicated that there were no morphological differences, in terms of histological structure and volume density, between the luteal tissue of SO heifers and the luteal tissue of non-SO heifers on day 7 of the oestrous cycle which could interfere with its endocrine function.