The effect on the tegument of adult Fasciola hepatica of incubation in the sodium ionophore monensin, the Na+/K+-ATPase inhibitor ouabain and ouabain pretreatment followed by monensin has been determined in vitro by scanning and transmission electron microscopy (SEM, TEM). With monensin incubation alone (1 x 10(-6) M), a flattening of the tegument with some loss of spines on the ventral surface is evident from 0.5 h onwards. Internally, the subtegumental musculature becomes grossly swollen, although there is no swelling of the infoldings of the basal plasma membrane of the tegument, even after 24 h incubation. Ouabain incubation (1 x 10(-3) M) induces folding of the apical surface of the tegument from 0.5 h onwards, and this is accompanied by the formation of blebs and microvilli. Brief (0.5 h) exposure to ouabain (1 x 10(-3) M) followed by monensin treatment (1 x 10(-4) M, 3 h) leads to gross "vacuolation" of the tegument, but this is not due to swelling of the basal infoldings. The other main feature of ouabain-pretreated flukes is the projection of basal lamina-like material into the tegumental syncytium. Monensin treatment alone (1 x 10(-6) M) results in the Golgi complexes of the tegumental cells becoming very diffuse from 1.5 h onwards, and relatively few secretory bodies are present in the cytoplasm. After 0.5 h incubation in ouabain (1 x 10(-3) M), the Golgi complexes of the tegumental cells are indistinct, although numerous secretory bodies are still present. The classical monensin-induced swelling of the Golgi cisternae is observed in the tegumental cells only when monensin treatment (1 x 10(-4) M, 3 h) was preceded by brief (0.5 h) exposure to ouabain (1 x 10(-3) M). The results are discussed in relation to the postulated osmoregulatory role of the tegument and the role of sodium pumps in membrane function in the fluke.