The pharmacodynamic effect of ceftriaxone on smooth muscles was investigated in isolated organs. Maximum stimulation of isolated guinea pig's ileum, rabbit's duodenum and rat's fundic strip was achieved by addition of 1024 µg of ceftriaxone/ml bath. While in isolated rat's colon, it was achieved by 512 µg of ceftriaxone/ml bath. The effect of graded increased concentrations of ceftriaxone on isolated rat's uterine muscles was examined during various stages of sex cycle. Ceftriaxone in the tested concentrations produced a dose-dependant negative inotropic effect on isolated rabbit's heart and guinea pig's auricles. Ceftriaxone in all tested concentrations did not induce any effects on the resting tonus of isolated guinea pig's tracheal chain and rabbit's aortic strip. Neuromuscular blockade effect was investigated on isolated frog's gastrocnemius muscle and frog's rectus abdominis muscle preparation. It was concluded that, ceftriaxone directly stimulates the smooth muscles of gastrointestinal tract and depresses those of uterus as well as cardiac muscles. These findings indicated that ceftriaxone had scarcely any pharmacological properties which might be leading to severe adverse reactions in clinical use.