In experiments on guinea pig sciatic nerve-tibialis anterior muscle preparations, with the use of suprathreshold stimulation, it was found that neostigmine enhanced the strength of the contraction. If an apprnprí ate interval between doses was used, the facilitative effect of neostigmine was cumulativé. There was nearly a fourfold increase in strength of contraction after the third dose of neostigmine. The administration of colistin, an antibiotic possessing some curarelike properties, did not modify the amplitude of the muscular contractions when intravenous doses up to 250 mg/kg were used. If neostigmine was administered not later than 5 injection of colistin, its facilitative effect on the muscular contraction was only slightly diminished; however, if 10 to 20 min elapsed between the administration of the two drugs, the effect of neostigmine was entirely antagonized by colistin. Cumulativé doses of neostigmine also failed to have an effect. The nature of the antagonistic effect of colistin and its slow appearance is discussed
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