To evaluate whether or not heart-rate variations (HRV) have both periodic and deterministic-chaotic properties in conscious and anaesthetized states, HRV of 9 rabbits were analysed by power spectral analysis and methods of deterministic chaos. In order to correlate these properties with the physiological structure of HRV, cholinergic and beta-adrenergic blockades and medullar transections were done. A predominant deterministic-chaotic structure of HRV with high fractal correlation dimension (CD) during conscious state changes to a more periodic structure and lower CD during general anaesthesia. The main periodic and deterministic-chaotic components of HRV are vagally mediated because of their strong reduction by cholinergic blockade, but not by beta-adrenergic blockade or medullar transection.