1. ZENECA ZD7288 (4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride, formerly ICI D7288) is a novel sino-atrial node function modulator which selectively slows heart rate. 2. The haemodynamic effects of ZD7288 (0.1, 0.3 and 1.0 mg kg-1, i.v.) have been evaluated and compared with those of placebo (physiological saline), zatebradine (ULFS 49, 0.1, 0.3 and 1.0 mg kg-1, i.v.) and propanolol (0.03, 0.1, and 0.3 mg kg-1, i.v.) in beagles chronically instrumented for measurement of heart rate, aortic pressure, aortic flow and dPLV/dtmax. The dogs were trained to run at 6.5 k h-1 on a level treadmill for 5 min at half hourly intervals over a period of 4 h. Drugs were dosed cumulatively after the second, fourth and sixth exercise periods. 3. Control experiments demonstrated a degree of accommodation to repeated exercise over a period of 4 h. Resting heart rate decreased by 21 beats min-1, but heart rate response to exercise was maintained, whereas dPLV/dtmax at rest remained steady while the response to exercise decreased significantly (by 25% after 2 h, P < 0.05). 4. ZD7288 and zatebradine both decreased heart rate during exercise in a dose-dependent manner, whilst heart rate at rest did not differ from resting heart rates in saline dosed control animals. In contrast, heart rate at rest and during exercise were lowered equally by the lowest doses of propranolol (approximately by 30 beats min-1), and additional doses caused only minor additional decreases. The exercise-induced tachycardia was maintained within 12% of pre-dose levels, presumably by withdrawal of vagal tone.5. Cardiac inotropism, as indicated by dPLv/dt max, was not affected by ZD7288 or zatebradine at rest,although the inotropic response to exercise decreased in proportion to the decreases in exercise-induced tachycardia. Propranolol caused a marked dose-dependent decrease in the exercise-induced inotropic response (by 85% at 0.3mg kg-1).6. Whilst the sino-atrial node modulators increased stroke volume at rest, and augmented increases in response to exercise, propranolol did not affect resting stroke volume and decreased the responses to exercise.7. Cardiac output at rest and cardiac output increases during exercise were well maintained in the presence of ZD7288 and zatebradine in contrast to propranolol which induced a significant depression of cardiac output, both at rest and during exercise. Propranolol also caused significant systemic vasoconstriction.8. In conclusion, ZD7288 has haemodynamic actions comparable to those of zatebradine despite their chemical dissimilarity. ZD7288 may be of benefit in the treatment of ischaemic heart disease by reducing heart rate without impairing cardiac function.