The analysis of heart rate variability (HRV) provides information on neural control of the heart. We investigated HRV in normal rats and in models of experimental cardiac hypertrophy using the Holter monitoring and peak/trough method. In normal rats, two heart rate oscillations with different wavelengths, high frequency (HF) and low frequency (LF) oscillations, were detected. The HF oscillations were insensitive to propranolol and suppressed by atropine. The LF oscillations were sensitive to both antagonists. Thyrotoxicosis resulted in cardiac hypertrophy (+20%) and tachycardia. The HF oscillations were unchanged, whereas LF oscillations were hampered at low heart rate in this group. Aortic stenosis resulted in cardiac hypertrophy (+53%), but heart rate oscillations were unchanged. The (number x amplitude) product for both types of oscillations correlated with heart rate in controls but not in the thyrotoxicosis or aortic stenosis models. Alterations of HRV in cardiac hypertrophy occur in rats as in humans. They may reflect the changes in the molecular components of the adrenergic/muscarinic system, which defines the new myocardial phenotype.