The aim of this study is to establish the effect of different concentrations of clove oil on the time needed for induction and recovery from anaesthesia of two different weight groups one-summer-old wels. Experiments were made with the following four concentrations: 0.02 ml.l-1, 0.04 ml.l-1, 0.06 ml.l-1 and 0.08 ml.l-1 with the two weight groups (WG) reared up to one-summer-old age in earthen ponds. 1. Weight group I (WG I) – fish weighing 11.92 ± 2.46 g. 2. Weight group II (WG II) – fish weighing 128.2 ± 1.65 g. The behavior of the fish was observed and analyzed during the experiment. At lowest concentration the wels from both weight groups do not reach the phase of complete immobilization. Individuals from WG I reach only phase 3 without going into phase 4 of anaesthesia, and with the fish from WG II only a slight decrease in locomotor activity is observed. The application of the next two concentrations 0.04 ml.l1 and 0.06 ml.lthere is a significant time difference between the two weight groups for reaching the same phase of sedation. At the highest concentration of 0.08 ml.l-1 the wels from both weight groups lose equilibrium in the shortest time and in the shortest time reach phase 4 – total loss of reactions. LC for the two weight groups was determined. For WG I 50% mortality rate was observed when the wels were exposed to the clove oil for a period of ten minutes at a concentration of 0.14 ml.l-1, and for WG II at a concentration of 0.16 ml.l-1