By limiting filling, abnormalities of right ventricular (RV) diastolic function may impair systolic function and affect adaptation to disease. To quantify diastolic RV pressure-volume relations and myocardial compliance (MC), a new sigmoidal model was developed. RV micromanometric and sonomicrometric data in alert dogs at control (n = 16) and under surgically induced subacute (2-5 wk) RV pressure overload (n = 6), volume overload (n = 7), and ischemia (n = 6) were analyzed. The conventional exponential model detected no changes from control in the passive filling pressure-volume (P(pf)-V) relations. The new sigmoidal model revealed significant quantifiable changes in P(pf)-V relations. Maximum RV MC (MC(max)), attained during early filling, is reduced from control in pressure overload (P = 0.0016), whereas filling pressure at maximum MC (P(MCmax)) is increased (P = 0.0001). End-diastolic RV MC increases significantly in volume overload (P = 0.0131), whereas end-diastolic pressure is unchanged. In ischemia, MC(max) is decreased (P = 0.0102), with no change in P(MCmax). We conclude that the sigmoidal model quantifies important changes in RV diastolic function in alert dog models of pressure overload, volume overload, and ischemia.