In this review, the basic mechanism of the parasympathetic nervous effect on the heart is discussed. This is expanded to clinical situations to clarify what can happen to patients after cholinesterase (ChE) inhibitor is administered and to avoid unnecessary adverse effects. The parasympathetic nervous system can affect heart as well as brain function, and its effect on the heart is more complicated than is generally thought. The best-known effect is the cardioinhibitory effect, i.e. slowing of the heart rate. Its effect is also very sensitive to the time at which the stimulus falls within the cardiac cycle (phase-dependent effect). On some occasions, a cardiostimulatory effect can be observed. The parasympathetic nervous system also interacts with the sympathetic nervous system (sympathetic-parasympathetic interaction). ChE inhibitors or acetylcholinesterase inhibitors are often being administered to improve cognitive function of patients with Alzheimer's disease. The heart is naturally rich with ChE, and its inhibition may affect cardiac function, especially in elderly patients, many of whom have concomitant cardiovascular disease. Inhibition of ChE retards ACh degradation and potentiates the cardioinhibitory effect. However, the effect of ChE inhibitor is only slight in patients that receive a typical dose. After administration of ChE inhibitor in humans, the phase-dependent effect is reduced because the parasympathetic nervous effect is potentiated and saturated (saturation mechanism). Beat-by-beat fluctuation is reduced. ChE inhibitor increases arterial blood pressure through central M1 and M2 subtypes of muscarinic receptors (Br J Pharmacol 127:1657-1665, 1999). However, diastolic blood pressure increases slightly.