The haemodynamic effects of the Ca-agonist BAY K 8644 alone and in combination with the selective beta-I-adrenoceptor agonist dobutamine were studied in the isolated rabbit heart. BAY K 8644 produced a concentration-dependent increase in contraction amplitude and shortening velocity, oxygen consumption and heart rate. In combination with a small fixed concentration of dobutamine (40 nM), Bay K 8644 produced similar alterations. When Bay K 8644 was infused at a fixed concentration (38 nM), dobutamine likewise produced similar increments in contraction amplitude, shortening velocity and heart rate, whereas oxygen consumption was considerably argumented. Both BAY K 8644 and dobutamine showed definite positive inotropic effects in the isolated rabbit heart. Combination of the two drugs did not yield a stronger positive inotropic effect than that seen on single drug administration, and oxygen consumption was even increased.