Compounds 3a [1-(3′,4′-methylenedioxyphenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline] and 3b [1-(6′-bromo-3′,4′-dimethoxyphenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline] demonstrated dose-dependent (5-75 μM) negative inotropic effects. Compound 5 [1,3-bis-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin- 1-yl)propane] demonstrated a two-phase inotropic effect. The experimental results and a literature analysis suggested that the negative inotropic effects of the tested compounds (3a, 3b, 5) could be due to their influences on the [Ca2+]i concentration through blocking of cardiomyocyte Na+- and Ca2+-channels. The positive inotropic effect of free base 5 could be related to modulation of cardiomyocyte muscarinic receptor activity.