Exercising horses have extremely high right and left atrial pressures. Limitation in ventricular function (i.e. relaxation) may play a role in these high pressures. We studied relaxation characteristics of the right ventricular myocardium and the impact of frusemide (2.0 mg/kg bwt i.v.) on these characteristics in horses exercising at 8, 10, 12 and 14 m/s. Exercise tests were performed 4 h after administration of frusemide. Right ventricular (RV) pressure was analysed using Fast Fourier Transform techniques to remove non cardiac components of the pressure signal. Mean right atrial (RA) pressure increased with exercise and was significantly attenuated at all speeds by frusemide. RV maximum and minimum rates of pressure change with respect to time (RV + dP/dtmax, RV-dP/dtmax) increased with exercise and RV relaxation time constant (RV tau) and time of RV relaxation from 65-20% of the difference between maximum and minimum ventricular pressure (delta 65-20) decreased with exercise. Frusemide produced no significant differences in +dP/dtmax, -dP/dtmax, RV tau or delta 65-20 except at 12 m/s where RV tau was longer after frusemide (23.4 ms for frusemide vs. 19.7 ms for control). Significant reductions in stroke volume were seen at 8, 10 and 14 m/s after frusemide. These results suggest that the reduction of atrial pressure by frusemide is not due to changes in ventricular relaxation rate.