Doramectin was used in a strategic programme for the prevention of parasitic gastroenteritis in first season grazing calves. Three groups of nine calves were used: group 1 was left untreated, group 2 was treated with doramectin at 0.2 mg/kg at turnout and again eight weeks later, and group 3 was treated with 0.2 mg/kg ivermectin at three, eight and 13 weeks after turnout. Both treatment programmes prevented the gastroenteritis which occurred in the controls. The growth rates of the treated calves were superior, and their faecal egg output, and serum pepsinogen and gastrin concentrations were all substantially lower than those of the control calves. The numbers of Ostertagia species larvae on the pastures grazed by the treated calves were also lower than on the pastures grazed by the control calves.