The effects of selective A₁ receptor agonist on human spermatozoa were examined to verify physiological responses and to investigate the signal transduction pathway. N⁶-Cyclopentyladenosine on uncapacitated spermatozoa did not induce spontaneous acrosome reaction after 5 h capacitation, whereas the number of capacitated spermatozoa, assessed by lysophosphatidylcholine-induced acrosome reaction with Pisum sativum agglutinin staining, was significantly increased. N⁶-Cyclopentyladenosine was also added to capacitated human spermatozoa to find out whether the agonist could induce the acrosome reaction. Results, although statistically significant, could not be considered biologically significant. A₁-Mediated capacitation was followed by the increase of tyrosine phosphorylation of a protein subset ranging between M_r = 200 000 and 30 000. Stimulation of A₁ receptor with the selective agonist elicited an agonist-induced inositol phospholipid hydrolysis leading to a transient rise of inositol triphosphate (IP₃). This increase was not induced by A₁ receptor antagonist and was blocked by phospholipase C inhibitor. Coimmunoprecipitation experiments showed that the A₁ receptor is coupled to Gα_i2 subunit suggesting that the activation of phospholipase C is mediated by βγ subunits. In conclusion, the A₁ adenosine receptor in human spermatozoa is coupled to Gα_i2, signals via IP₃, and affects the capacitative status of ejaculated spermatozoa.