Capacitation and the acrosome reaction of mammalian spermatozoa are essential for fertilization. In vitro results are presented that demonstrate that catecholamines stimulate activation (a whiplash flagellar movement characteristic of capacitated hamster spermatozoa) and the acrosome reaction. Protein-free ultrafiltrates of bovine adrenal cortex and medulla preparations stimulated motility, activation, and acrosome reactions of hamster spermatozoa in the presence of bovine serum albumin. The medulla preparation was more effective than the cortex preparation in the stimulation of activation and acrosome reactions. Epinephrine (0.5-50 μM) and norepinephrine (50.0 μM) in the presence of bovine serum albumin and a partially purified protein-free cortex preparation also stimulated activation and the acrosome reactions. Both activation and acrosome reactions in the presence of epinephrine were inhibited by the adrenergic antagonists phentolamine and propranolol, suggesting the involvement of α- and β-adrenergic receptors in the stimulation of capacitation and the acrosome reaction. In addition, phenylephrine, an α-adrenergic agonist, was as potent as epinephrine in the stimulation of acrosome reactions, but activation was reduced. Isoproterenol, a β-adrenergic agonist, was as potent as epinephrine in the stimulation of activation, but acrosome reactions were reduced. High percentages of both activation and acrosome reactions were observed only in the presence of epinephrine, norepinephrine, or phenylephrine and isoproterenol together.