1. Patch clamp techniques were used to measure the ionic dependence of amiloride block of single mechanosensitive (MS) channels in frog (Xenopus laevis) oocytes. 2. The primary aim was to determine whether the difference in potency of amiloride block of MS channels in frog oocytes (IC50 = 0.5 mM) and chick auditory hair cells (IC50 = 50 microM) was due to the different ionic recording solutions. 3. Amiloride block of the oocyte MS channel does not vary significantly with complete substitution of external Na+ (i.e. 100 mM) with K+ in Ca(2+)-free recording solution (in both Na+ and K+ the IC50 = 0.5 mM). 4. A physiological concentration (1.8 mM) of external Ca2+ blocks the oocyte MS channel and reduces the potency of amiloride block (IC50 = 1.1 mM) without altering the voltage-dependence or the HIll coefficient (n = 1.8) of amiloride block. The reduction in potency can be explained by surface charge screening by Ca2+ which reduces the effective amiloride surface concentration. 5. The present results indicate that factors other than ionic recording conditions must underlie the difference in potency of amiloride block of MS channels in oocytes and auditory hair cells.