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 paM) and norepinephrine (50.0 pM) 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 a-and a-adrenergic receptors in the stimulation of capacitation and the acrosome reaction. In addition, phenylephrine, an a-adrenergic agonist, was as potent as epinephrine in the stimulation of acrosome reactions, but activation was reduced. Isoproterenol, a P-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.Sperm capacitation (subtle cellular changes occurring within the female reproductive tract or in vitro) and the resulting acrosome reaction (a fusion and vesiculation of the outer acrosomal membrane and its overlying sperm head plasma membrane) are essential for mammalian fertilization (1, 2). The molecular events of capacitation and of the acrosome reaction, including its initiation, are only partially understood (3). Capacitation and the acrosome reaction can be induced in hamster spermatozoa in vitro by detoxified follicular fluid (4,5) or by blood sera (6). A nondialyzable serum or follicular fluid factor (labile at 900) was required for the acrosome reaction to occur and a dialyzable heat-stable factor derived from serum or follicular fluid was necessary for sperm motility, activation (the whiplash flagellar movement characteristic of capacitated hamster spermatozoa), and the occurrence of the optimum number of acrosome reactions (4-6). It has now been shown that certain serum albumins can replace the nondialyzable factor (6-9) and that the bovine follicular fluid factor is serum albumin (9).Recently, it has been reported that the dialyzable factor can be replaced by a "motility factor" of less than 1000 molecular weight obtained from extracts of hamster adrenal gland (10) or hamster, guinea pig, or human spermatozoa (11,12). The motility factor derived from spermatozoa also seems to maintain hamster sperm viability upon dilution in vitro (12). In the present paper the bovin...