1. The hypotensive activities of the synthetic colistin derivatives in which the amino groups were substituted for N-alkyl and N-alkylidene in the position of colistin base were studied in dogs, cats and guinea pigs. The hypotensive activity of colistin derivatives was proportional, both in intensity and duration, to the number of carbon atoms in N-alkyl and N-alkylidene groups. The decreasing order of hypotensive activity of colistin derivatives is as follows: N-amyl>N-butyl>N-propyl>N-ethyl>N-methyl and N-benzyl>N-butylidene>N-propylidene>N-ethylidene>N-methylidene.
2. Except for N-amyl colistin, N-alkyl and N-alkylidene produced negative inotropic and chronotropic effect on the toad's isolated heart. The decreasing order of the negative inotropic action of colistin derivatives is as follows: N-benzyl>N-butylidene>N-propylidene>N-ethylidene>N-methylidene and N-butyl>N-propyl>N-ethyl>N-methyl.
3. N-alkyl and N-alkylidene had vasoconstrictor effect on the toad's perfused hind limbs. The decreasing order of the vasoconstriction intensity of these derivatives is as follows: N-butyl>N-butylidene>N-propylidene>N-ethylidene>N-methylidene.