The in vivo efficacy of monensin against Fasciola hepatica was determined in the albino rat. The results were variable, with monensin generally showing greater activity against juvenile (two-week-old) than adult (12-week-old) flukes. Significant (p less than 0.005) reductions in worm burdens were obtained only following treatment of adult flukes with 2 X 10 mg/kg monensin (52.9% efficacy), and of juvenile flukes with 1 X 10 mg/kg (42.4% efficacy) and 2 X 10 mg/kg (56.23% efficacy). Monensin administered in the diet (200 ppm) had a negligible effect on juvenile and adult fluke burdens. Prophylactic treatment of rats with monensin (100 ppm) produced a 45.5% efficacy, but this was not statistically significant. At doses of 1 X 5 mg/kg and 2 X 2.5 mg/kg, monensin had little effect on egg output by F. hepatica. No clear relationship was established between egg output and worm burden, and so faecal egg counting was not a reliable indicator of fluke burdens in the rat.