Octopamine, beta-phenylethylamine, and tryptamine produced a dose-dependent negative inotropic effect as shown by reduced cardiac output (IC(50)=109 microM, 159 microM, and 242 microM, respectively). In the same preparation a similar effect was produced by thyronamine and 3-iodothyronamine, with IC(50)=94 microM and 27 microM, respectively.