A mathematical model describing the relationship between plasma concentrations of oxprenolol and submaximal-exercise heart rates has been developed on the basis of data available from 34 healthy volunteers. This relationship, which is exponential in type, is consistent with the 'law of diminishing returns', and is independent of the pharmaceutical formulations used; it appears to be established instantaneously, and is not modified by repeated drug administration over 8 days. At oxprenolol concentrations of 190-250 ng/ml, between half and three-quarters of the subjects investigated reached 90% of their estimated maximum levels of beta-adrenoceptor blockade.