Several alpha-adrenergic antagonists inhibited the activation of calmodulin-stimulated phosphodiesterase at concentrations that had little or no effect on basal phosphodiesterase activity. The most potent of these compounds were phenoxybenzamine and dibenamine (IC50 values of about 1 microM); the amino acid ergot alkaloids ergocryptine, ergocristine, ergotamine and their dihydrogenated derivatives were less potent calmodulin-inhibitors (IC50 values of 35-80 microM). The amino ergot alkaloids ergonovine and methysergide were essentially devoid of inhibitory activity. A variety of other alpha 1-antagonists (phentolamine, tolazoline and prazosin), an alpha 2-antagonist (yohimbine), alpha-agonists (norepinephrine, phenylephrine and clonidine), beta-adrenergic antagonists (propranolol and practolol) and the beta-adrenergic agonist methoxyphenamine displayed little or no anti-calmodulin activity (IC50 values greater than 300 microM). Similarly, the alkylating agents chlorambucil and mechlorethamine also failed to inhibit calmodulin activity. Phenoxybenzamine and dibenamine inhibited calmodulin activity irreversibly, whereas the inhibition caused by other alpha adrenergic blocking agents was reversible. Phenoxybenzamine inhibited calmodulin activity by binding directly to it. This binding was calcium-dependent and irreversible. The irreversible binding and inhibition of calmodulin activity by phenoxybenzamine (or dibenamine) may serve as a useful tool for studying the sites at which drugs bind to calmodulin and may also be useful for studying the distribution and turnover of calmodulin.