The mechanosensitivity of eel (Anguilla anguilla) neuromasts was measured by the impulse responses of single afferent nerve fibers to mechanical stimuli. It is dependent on the potential across the skin and on the ions in the water outside the apical membrane of the sensory cells. The mechanosensitivity decreases to zero when the skin is polarized by 10-100 mV cathodal DC (skin surface negative); it increases with increasing (10-60 mV) anodal DC and remains remarkably constant with higher polarization (Fig. 1). The mechanosensitivity increases with increasing concentrations of Ca++ outside the apical membrane of the sensory cells. Na+ and K+ have no influence. Addition of La , Co++, Mg++, D 600 and A-QA 39 inhibits the mechanosensitivity; the degree of inhibition varies with the inhibitor and the ratio [Ca++]/[inhibitor], indicating that the inhibition is competitive (Figs. 2, 3). We conclude that the apical membrane is specifically permeable to Ca++ ('late Ca channel') and that the inward receptor current through the apical membrane is carried by Ca++. Streptomycin also inhibits mechanosensitivity by competing with Ca++. With streptomycin, however, anodal polarization reduces, rather than increases, the mechanosensitivity (Fig. 4).(ABSTRACT TRUNCATED AT 250 WORDS)