1. Ambulatory (24 h) cardiac monitoring (ACM) is frequently used to screen healthy volunteers before inclusion in trials of new chemical entities in man. We analysed 156 consecutive ACM recordings in 'healthy' volunteers (on no medication). 2. Only 20 (13%) of the recordings showed normal sinus rhythm throughout. 3. Supraventricular ectopics were the commonest abnormality (83%). Ventricular ectopics occurred in 11%; ventricular tachycardia (unsustained) in 2% and sinus pauses in 6.5%. One volunteer was found to be in atrial fibrillation throughout. 4. The data indicate that when ACM recordings are performed in the assessment of the effects of experimental drugs, guidelines are needed to assess 'normality' to suggest when cardiological investigation is needed and to assign causality of the arrhythmia to the new chemical entity. 5. Proposed guidelines are presented.