The effect of exogenous steroid hormones on the egg output of Heligmosomoides polygyrus (Nematoda: Trichostrongylidae) was examined in vitro. Using worms raised in female mice, it was found that estradiol, testosterone, and cortisone each significantly decreased egg output. Although similar trends were found using H. polygyrus raised in male mice, none of the decreases found was significant. No significant differences were found with ecdysone or progesterone treatments using worms from female or male mice. Treatment of worms with cortisone did not significantly affect retention of eggs within the uterus of H. polygyrus. Titration of the effect of cortisone on egg output indicated that levels of reduction were significant for concentrations of 5.6 x 10(-6) M to 5.6 x 10(-3) M in worms from female mice and for concentrations of 5.6 x 10(-8), x 10(-7), x 10(-5) and x 10(-3) in worms from male mice. Radioisotope labelling experiments showed incorporation of 3H-corticosterone in the nucleus of intestinal cells of H. polygyrus suggesting that its effect on egg production may be via a modulatory effect on the intestinal cells.