The pD2-values for noradrenaline obtained from the veins of the body wall were quite different from those embryogenetically related to the digestive tract. Therefore, the difference in the post-junctional alpha 1-adrenoceptor mechanism was studied in venous smooth muscle preparations from the canine. The helical and longitudinal strips of the lateral saphenous, femoral and portal veins, and the inferior vena cava were prepared and set in an organ bath apparatus. The longitudinal strips of the lateral saphenous and femoral veins, and inferior vena cava did not or little respond to noradrenaline. The pD2-values for noradrenaline differed considerably among the veins, while the pA2-values for prazosin against noradrenaline were identical in all veins used. Negative log of dissociation constants, pKA-values, for noradrenaline obtained by the partial irreversible blockade of alpha 1-adrenoceptors with phenoxybenzamine were very similar in all the veins. Efficacies which were calculated by the same method differed considerably among the veins. The dissociation constants, KD-value and the maximum binding sites, Bmax-values for [3H]-prazosin were also estimated by Scatchard analysis of the specific binding of [3H]-prazosin to the microsomal fractions from veins. The KD-values for [3H]-prazosin were also identical. However, Bmax-values varied considerably among the veins. The Bmax-values are proportional to the 50% effective concentrations, whose negative logs were the pD2-values, and to the efficacies. The present results suggest that the regional difference in the pD2-values for noradrenaline in the canine veins is not due to the affinities to the alpha 1-adrenoceptors but to the receptor densities.