The authors previously reported thatd-tubo-curarinc (dTc) had a negative inotropic effect on isolated perfused rabbit hearts; the effect was concentration-dependent and related to myocardial calcium exchange. This paper reports results of a concentration–response study of die effects of crystalline dTc, various commercial preparations of dTc, and their preservatives on the contractile amplitude of stimulated rabbit atrial strips. Crystalline dTc produced a slight positive inotropic effect which was significantly different from control, while the commercial preparation, Tubarinc (used in previous studies), produced depression with 43 ± 10-5to 5.4 X 10-4[M]. This depression closely paralleled that produced by antibacterial preservative, p-chloro-m-cresol, while the antioxidant preservative, K mctabisulfite, produced minor depression. Other commercial preparations containing antibacterial preservatives also produced depression similar to the depression produced by their antibacterial preservatives. The commercial preparations dispensed without antibacterial preservatives produced relatively little depression. The authors conclude that their previous results were due to the preservative, p-chloro-m-cresol, and not to dTc