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The aim of this study was to evaluate ovarian activity and the size of oocytes in ovarian follicles of sexually mature Landrace-Yorkshire gilts in relation to the seasons of the year. The size and weight of the ovaries, the number of follicles and corpora lutea (CL) according to individual size categories were evaluated on 240 gilts slaughtered at an abattoir during the four seasons of the year. Our evaluation of the ovary size showed that they were the largest in autumn when their mean length reached 25.8 ± 3.4 mm, whereas in winter their mean length was 24.2 ± 2.9 mm (P < 0.05). During the autumn months, the largest number of follicles in the ovaries of the gilts was recorded, with a predominance of follicles up to 3 mm (mean number 17.9 ± 7.5). The smallest number of corpora lutea was observed in winter (mean number 6.1 ± 1.1) and the largest in spring (mean number 12.1 ± 2.6). The oocytes, from follicles of up to 3 mm size, were the smallest in spring (mean size 16.99 103 ± 3.42 103 µm2). In total, the largest oocytes were aspirated from 4--6 mm follicles in autumn (mean size 19.60 103 ± 5.37 103 ± µm2). The findings of the study indicate that the ovarian activity and growth of oocytes in gilts are affected by the seasons of the year. [ABSTRACT FROM AUTHOR]