CSL has continued the development of its Cardiac Scintillation Probe System begun in 1977. This non-imaging ECG-gated scintillation probe, when used in conjunction with left ventricular (LV) catheterization, permits simultaneous quantification of the variation of LV volume and pressure. By simultaneously measuring LV volume and LV pressure, parameters such as LV compliance can be continuously monitored, in addition to such measurements as ejection fraction, filling and ejection rates, and temporal relationships. This year the previously implemented capability for realtime pressure-volume measurements were used to study the effects of nephidipine and verapermil on patients with asymmetric septal hypertrophy. The pressure-volume relationships produced by the probe system allowed the effects of drugs to be quantitated in a manner not possible before. New hardware and software have been developed to allow online calculation of new parameters and to permit the system to be easily used on a routine basis by the Clinical Center personnel. Development is continuing on increasing the detection efficiency of the probe and in quantifying the limitation of the technique.