1. 9-Amino-1,2,3,4-tetrahydroacridine (THA) is a compound with structural similarity to the K+ channel blocker 4-aminopyridine. It was investigated for its effects on myocardial membrane currents in guinea-pig isolated ventricular myocytes. 2. THA prolonged the transmembrane action potential and decreased the amplitude of its plateau in a reversible manner. 3. Voltage-clamp experiments showed that THA reduced the inwardly rectifying and the time-dependent outward K+ currents as well as the slow inward Ca2+ current. 4. The degree of block of the inwardly rectifying K+ current depended on the membrane potential, being more pronounced at more positive potentials.