SummaryDaily changes in serum gastrin and pepsinogen concentration have been studied during two types of infection withOstertagia ostertagiin calves. In a first experiment two calves were trickle infected (10 times 10,000 L3Ostertagia) and two animals received a single infection of 100,000 L3Ostertagia. Gastrin and pepsinogen changes are discussed in relation to adult wormburdens.The second experiment involved 8 calves and was designed to investigate pepsinogen and gastrin changes following a challenge infection in previously sensitized calves.The high dosed group was infected with 5,000 L3O. ostertagiduring 30 days, the low dosed group received 500 L3O. ostertagiand group 3 served as uninfected control. At day 41 post infection all animals were treated with oxfendazole and on day 61 challenged with 100,000 L3O. ostertagi. Only in the high dosed group a distinct pepsinogen and gastrin reaction was noticed. Both parameters dropped to almost preinfection levels after treatment. Two days post challenge a moderate rise (± 1,000 mU tyr) of the pepsinogen concentration was observed in the previously infected animals and gastrin showed a temporary slight increase in several animals 8 to 10 days post challenge. The effect of treatment and challenge infection is discussed in relation to gastrin and pepsinogen changes and immunity