Objective. To adopt and validate the Straub tail reaction (SR) for comparative assessment of spastic effects of serotonergic compounds. Material and methods. To measure the muscle relaxant activity, the morphine-induced Straub-tail assay was used. SR was graded according to modified intensity-score basis in a scale decribed by Kameyama et al. (1978). Subcutaneous injections of different doses of morphine (10—60 mg/kg) induced a dose-dependent SR with maximum response obtained 15—30 min after the morphine administration. Results and conclusion. The centrally acting muscle relaxant baclofen (3—10 mg/kg) reduced SR induced by morphine (40 mg/kg) at all used doses; tizanidine decreased the intensity of SR at highest doses tested (0.6 and 1 mg/kg). Dantrolene (20—100 mg/kg), a peripherally acting muscle relaxant, did not affect SR. Effects of serotonergic agents depended on the specific mechanism of action. SR appears to be available for rapid evaluation of the effect of antispasticity drugs.