The apoptosis in the course of experimental infection with T. pseudospiralis in mice. The level of apoptic cells in the lamina propria of the mucosa of the jejunum and in the masseter muscle in the mice infected with 200 larvae T. pseudospiralis was observed. Cryostat preparations made from jejunum and muscle samples on 7, 14, 21, 28, 60, 180 and 360 day post infection (dpi), were examined employing the TUNNEL method with the use of "In situ Cell Death Detection KIT Fluorescein" of Roche. The number of apoptic cells in the lamina propria of jejunum started to increase as early as on 7 dpi. The highest number of the apoptic cells, on an average - 56,4 in the crypta-villus unit, was recorded on 28 dpi, whereas the return to the standard level took place only 360 dpi. The level of the apoptic cells in the masseter muscle was the highest on 28 and 60 dpi, which constituted some 30% of all the cells of infiltration.