1. The aim of the present study was to investigate an eventual influence of the hour of administration on lidocaine kinetics in the rat. 280 Wistar AF SPF adult male rats were used for this study and maintained under controlled environmental conditions (LD: 06.00-18.00) during the month of October. A single 50 mg . kg-1 dose of lidocaine was given by intramuscular route, at four different fixed time points of a 24-hour period (i.e.: 10.00, 16.00, 22.00 and 04.00) to 70 rats. At each specified time point blood samples were taken 5, 15, 30 min., 1, 2, 4 and 6 hours after the drug administration. Lidocaine plasma levels (free and bound) were determinated according to a specific gas chromatographic method. 2. Our data showed circadian variations of pharmacokinetic parameters (Formula: see text) 3. Lidocaine free fraction varied with time and the protein binding of lidocaine showed a circadian variation. 4. The observed variations may be related to 1) daily fluctuations of absorption or binding of the drug 2) to diurnal variation of the hepatic drug metabolising enzymes responsible for the inactivation of the drug and/or 3) to diurnal variations in excretion rate of lidocaine. Finally our results agree with those of Lutsch and Morris who found a circadian periodicity in susceptibility to lidocaine in the mouse with maximal convulsant activity at 21.00.