Juvenile specimens of Lymnaea stagnalis were exposed to 0 or 4 miracidia of Trichobilharzia ocellata. Dissection followed at day 22 post exposure. The effects of infection on the activity of the female gonadotropic hormone producing Dorsal Bodies (DB) were studied by phase-contrast and electron microscopy. Morphometry shows that the relative volume of DB cells of infected snails is 1.6 times as high as in controls. This is mainly due to a 45% higher cytoplasmic volume. The number of profiles of the Golgi apparatus increases by nearly 90% and the Golgi volume by 40% as a result of infection. Numerous omega-shaped indentations of the plasma membrane of the DB cell processes indicate the release of the contents of the DB hormone containing elementary granules. It is concluded that parasitic infection causes a clear increase in the synthetic activity of the DB. Most probably, the parasites exert the inhibiting effects on reproductive activity at the level of the targets of the reproductive hormones.