The aim of this study was to evaluate the effect of anaesthesia (ether or ketamine) on daily rhythms of temperature (T), heart rate (H) and locomotor activity (A) in unrestrained rats by using implanted radio-telemetry transmitters. T, H and A were measured every 10 min, in Wistar male rats, and analysed using Cosinor. The mean +/- SEM days needed, after surgical implantation, to detect a daily rhythm in H, T and A were also assessed. Six rats were anaesthetized for about 50 min either by ketamine or ether in a 3 by 3 cross-over design. Mesors, amplitudes and acrophases of T, H and A were calculated three days before (D-3; D-2; D-1), the day of anaesthesia (D0) as well as the three following days (D1; D2; D3). ANOVA was performed in order to detect, firstly a possible effect due to the order of application of anaesthesia, secondly a significant difference between ether or ketamine-induced anaesthesia and finally a modification of the mesors, amplitudes and acrophases of T, H and A, induced by each anaesthesia, for D0, D1, D2 and D3 when compared to D-1. Our results indicate: (1) Alterations of the acrophases, mesors and amplitudes, except for the amplitude of A, of the daily rhythms of T, H and A on D0 of ketamine anaesthesia while regarding ether anaesthesia only amplitude of T and H and acrophase of A were modified on D0. Some of these modifications were still observed on the days following anaesthesia. A significant difference between ether and ketamine-induced anaesthesia was also observed. (2) A non-detection of T, H and A daily rhythms after surgical implantation, which was not observed after injection of either ether or ketamine alone. Almost 10 days were needed to detect a significant daily rhythm for T, H and A. The authors suggest that, the general anaesthetic agent was responsible for a perturbation of the mesors, amplitudes and acrophases of the daily rhythms of H, T and A while the non-detection of these rhythms after implantation was more due to the surgical aggression.