This study quantified the effects of a physiological masker, the heartbeat, on the detectability of a low‐frequency tone (100 Hz, 100‐msec duration). Using aYES‐NOsignal detection paradigm, binaural sensitivity (d′) was examined as a function of the temporal location of the signal onset within the cardiac cycle. The independent variable was the temporal delay in signal onset following the subject's own EKG R wave. Signal delays were selected (0.0 to 0.8 sec in 0.1‐sec steps) and presented to four subjects. Results reveal depressed sensitivity with signal delays of 0.0, 0.3, and 0.7 sec following the EKG R wave. In contrast, maximum sensitivity occurred near 0.5 sec following the R wave. The results were discussed in terms of physiological masking produced by valve closures within the heart.