Serum and antral gastrin were measured in rats infected with either Trichinella spiralis or Hymenolepis diminuta as a step in testing the hypothesis that parasites change certain aspects of host physiology by altering gastrointestinal (GI) hormone levels or responses to GI hormones. Parasitism with T. spiralis was associated with inflammatory changes in the small bowel mucosa and with a significant increase in serum gastrin. Neither changes in hormone level nor inflammation were induced in tapeworm-infected rats. These results reveal the capacity of tissue penetrating parasites to alter the level of circulating gastrin. This finding coupled with considerable indirect evidence suggests that some of the pathologic changes induced in hosts by enteric parasites may be due to changes in functions that are regulated by GI hormones.