The effects of different histamine concentrations (1 X 10(-6) to 1 X 10(-4)M) on the contractile effects of noradrenaline and acetylcholine are studied on isolated smooth-muscle preparations from rats. Histamine, applied in concentrations of 1 X 10(-5)M and 1 X 10(-4)M, markedly potentiates the contractile effects of the lower concentrations of acetylcholine on the anococcygeal muscle, without essentially changing the maximum contractions induced by it. Histamine reduces 6.5-31.6 times the acetylcholine concentrations causing 50 per cent of the maximal contractions of this smooth-muscle preparation. Application of histamine on anococcygeal muscle decreases contractile responses to noradrenaline. A dependence is found between the histamine concentration used and the shift to the right of the curve "log concentration-effect" for noradrenaline. Histamine increases 2.6 to 16.2 times the noradrenaline concentrations needed for inducing 50 per cent of the maximum contractile response of the anococcygeal muscle. The interaction between noradrenaline and histamine on this smooth-muscle preparation is not competitive. Histamine in the concentrations applied does not influence the contractile effects of noradrenaline on vas deferens and of acetylcholine on tracheal smooth-muscle preparation. In rat anococcygeal muscle histamine most probably influences the contractile effects of noradrenaline and acetylcholine through allosteric interaction with their receptors.