It is well known that both histamine and dimaprit show moderate histamine Hz-receptor agonistic activities on the guinea pig right atrium. Quantum chemical calculations on these two compounds showed similarities in electron distributions and molecular electrostatic potentials (MEP's), which could be extended. to rigid analogues [2-amino-5-(2-aminoethyl) thiaoles] of the latter structure. On the base of these results a series of substituted 4-and 5- ...