Bilateral electrocoagulation of the locus coeruleus (LC) were performed in 8 cats, in order to study the effects on the heart rate modulation during sleep. In 4 cats the lesions were restricted to the posterior part of LC: they induced a permanent tachycardia in all states, and a reduction of the phasic changes during paradoxical sleep (PS), while the state-linked modulations remained similar to those of controls. In another group of 4 cats, lesions extended rostrally to the anterior LC. They induced a bradycardia evidenced mostly during PS. Phasic changes persisted though the tachycardic component was greatly reduced. The state-linked modulations were opposite to those of controls: increase during transition from slow waves sleep (SWS) to PS and decrease during PS. No recovery trend was observed within the 3 weeks post-lesion experimental period. These data are discussed in relation to the well-known parasympathetic and sympathetic regulations of the heart rate, with special attention to the changes observed during transition from slow waves sleep (SWS) to PS and during the first minute of PS. This period corresponds to a shift in the balance of tonic and phasic influences.