Abstract 1. 1. The ionic response of the liver fluke, Fasciola hepatica to perturbation of Na,K-pump activity has been determined by atomic absorption spectrophotometry. 2. 2. The Na +/K +-ATPase inhibitor ouabain (1 × 10 −4M) induced a marked reduction in K + levels; Na + and Ca 2+ levels also fell. 3. 3. The sodium ionophore monensin (1 × 10 −4M) also caused a decrease in K + levels, to below that of Na +. 4. 4. Brief pretreatment with ouabain (1 × 10 −4 M, 15 min) followed by monensin treatment did not affect the decline in K + levels, but did prevent the short-lived Na + decline observed with monensin alone. 5. 5. The deacetylated (amine) metabolite of the fasciolicide diamphenethide caused a short-lived drop in Na + levels, but otherwise produced little change in ion levels within the fluke.