When incremental infusions of drugs that increase blood pressure are given to human subjects to assess pressor responsiveness, only the lower part of the sigmoid dose-response curve can be obtained. Fitting a quadratic function does not involve discarding data points, which is usually the case with a linear fit, and it provides a more satisfactory fit to the lower part of a sigmoid dose-response curve. In the presence of a competitive antagonist, a pressor dose-response curve will be shifted to the right. In this situation the dose-response curves obtained before and after treatment with antagonist should be fitted simultaneously to a quadratic model in which the parallel shift is one of the parameters. The use of quadratic fitting is illustrated by references to clinical experiments to obtain the following 3 curves for drugs that modify peripheral .alpha.-adrenoceptors: norepinephrine pressor response curves after placebo and doxazosin, an .alpha.1 antagonist; norepinephrine pressor response curves after placebo, labetalol and medroxalol (drugs with combined .alpha.1- and .beta.-blocking properties); and phenylephrine pressor response curves before and after prazosin. Fitting a quadratic function is the appropriate initial step in the analysis of pressor dose-response curves in man.