The aim of the study was to determine the relation between involution of the FZ and the proliferation and apoptosis of the adrenal cortex cells. The study was carried out with adrenal glands from guinea pigs aged 1, 3, 9, 14, 21, 35 and 90 days. Paraffin slices were stained with Mayer haematoxylin and eosin and with acid fuchsin. For the immunohistochemical reaction the Anti- -PCNA Clone PC10 (Dako) was used. Apoptosis evaluation was performed with use of the TUNEL method (Roche). The results support the theory of involution of the FZ cells realised mainly by way of apoptosis. Analysis of the distribution and number of PCNA-positive cells in the adrenal cortex on subsequent days of PD may also back up the migration theory of renewal of the adrenal cortex cells.