The binding affinities to various receptors in the development of novel drugs are generally evaluated mainly by the in vitro radioligand binding technique. However, the in vitro receptor binding characteristics of drugs may not necessarily reflect pharmacological specificity in vivo because it does not take various pharmacokinetics and pharmacodynamic factors into accout. Thus, the characterization of drug-receptor binding under physiological and pathological conditions would provide more practical information for the evaluation of novel drugs. In this article, we review in vivo receptor occupancy (degree, duration and tissue selectivity) by several 1,4-dihydropyridne (DHP) calcium antagonists and angiotensin II (AngII) receptor antagonists in different tissues, in relation to their pharmacokinetics and pharmacodynamics. It has been shown that there are considerable differences in the degree and duration of receptor occupancy by these drugs between tissues. In addition, the degree and time course of receptor occupancy in different tissues including the brain have been evaluated in relation to the plasma concentration and pharmacodynamic effects. Consequently, the data obtained under in vivo conditions, compared with those under in vitro conditions, appear to be more closely related to their pharmacodynamic effects. For the development of novel drug acting on receptors, therefore, analysis of in vivo receptor occupancy in different tissues would be very useful in predicting or characterizing their pharmacological specificity (potency, duration and receptor subtype or tissue selectivity) in relation to their pharmacokinetics.