A method of measuring online interval durations between different waves in the electrocardiogram is presented. After preprocessing two signals, data segments containing the wave segments of interest are sampled. The detection of time reference points for these waveforms is accomplished in a separate recognition phase (of prime importance for the reliability) and an estimation phase (mainly important for the method's accuracy). The recognition of a given wave segment is triggered by a pulse derived from the QRS complex by a hardware R-wave detector, to which the P-wave and T-wave are time related under normal circumstances. A recognition window is defined interactively by fixing a starting point and an end point in the sampled data segment. The estimation of wave positions is performed with the aid of four estimation criteria, which are described in the text. The capabilities of the method are discussed with reference to the P-P, P-R and R-R interval measurements under different experimental conditions.