The effect of the first presentation of a low intensity (20-50 dB SPL) 1000 Hz 2 sec tone upon sinus arrhythmia was investigated. Sinus arrhythmia was found to be severely disrupted following stimulus onset so that the conceptualisation of the HR OR as being superimposed upon a background of sinus arrhythmia appears incorrect. A comparison of temporal relationships between respiration and HR deceleration in both control and response periods confirmed this independence. Some refinement of the criteria for validating non-habituating responses with cardiovascular indices of the OR appears warranted. © 1978.