A capture detection circuit for an implantable cardiac stimulator. A signal detected by an electrode in the heart following delivery of a stimulating pulse is amplified, bandpass and highpass filtered, rectified, integrated over a selected window of time starting at a selected delay after delivery of the stimulating pulse, and applied to two comparators having different reference values. If the integrated signal exceeds the first reference value, the first comparator output goes high. If the integrated signal exceeds the second reference value, the second comparator goes high. If both comparators remain low, non-capture is indicated. If the first comparator goes high and the second comparator remains low, capture is indicated. If the second comparator goes high, an intrinsic contraction is indicated. The window of integration can be extended to distinguish capture from intrinsic contraction.