A study designed to investigate the relationship between the pharmacokinetics of digoxin and a measure of its pharmacological effect has been conducted. Serum digoxin concentrations and systolic time intervals were measured concurrently in 12 normal male volunteers following a 1.0 mg i.v. bolus injection. The averaged serum digoxin concentration--time and response--time data were analyzed pharmacokinetically using a three-compartment open model and nonlinear least-squares fitting. When only the serum level--time data were analyzed, a close relationship was found between calculated digoxin levels in the slowly distributing (deep) peripheral compartment and response of the heart to digoxin, as measured by changes in the QS2 index (delta QS2I). Although it was not possible to distinguish clearly a linear from a nonlinear relationship between digoxin levels in the deep compartment and delta QS2I, the nonlinear relationship gave the best overall fit when both serum digoxin and delta QS2I data were fitted simultaneously. The simultaneous fit yielded a total body clearance of digoxin of 3.6 ml/min/kg and a terminal t1/2 of 42 hr.