Intestinal goblet cell numbers in two regions of the small intestine (20-30% and 60-70% distance form the pylorus) of male, 6- to 8-week-old C57 mice have been monitored following a 5-cysticercoid infection of the rat tapeworm, Hymenolepis diminuta. Test and sham-infected control mice were autopsied 0, 4, 8, 10, 14, and 28 days postprimary infection (p-1 degree-i) and 2, 4, 5, 7, and 14 days postsecondary infection (p-2 degree-i), administered 28 days p-1 degree-i. Results show a statistically significant increase in the number of mucus-containing goblet cells in both regions of the intestine during primary and secondary infections. Peak goblet cell numbers occurred on Day 8 p-1 degree-i and Day 5 p-2 degree-i in the 20-30% region and on Day 10 p-1 degree-i and Day 5 p-2 degree-i in the 60-70% region. In both regions, cell numbers declined to control levels by Day 14 p-1 degree-i, but remained significantly above control values 14 days p-2 degree-i. The increase in cell numbers correlated with an increase in goblet cell theca size and observable amounts of luminal mucus. The same infection regime in mice treated with cortisone elicited no goblet cell response. Male Wistar rats given a 10-cysticercoid infection and autopsied on Day 0, Day 10, and 15 months p-i showed a statistically significant increase in mucus-containing goblet cells only in the 60-70% region of intestine 10 days p-i; however, the worm burden was not eliminated. The functional significance of these results is discussed in relation to host immunity and murine cestode rejection.