Binding to the zona pellucida of an egg stimulates the spermatozoon to undergo the acrosome reaction, a process that enables it to penetrate the egg. Before this binding, the spermatozoon undergoes a series of biochemical transformations in the female reproductive tract, collectively called capacitation. Only capacitated spermatozoa can bind to the zona pellucida and undergo the acrosome reaction. Protein kinases may be involved in the regulation of intracellular Ca2+ during capacitation and the acrosome reaction. The first event in capacitation is the increase in intracellular calcium, bicarbonate and hydrogen peroxide, which collectively activate adenylyl cyclase to produce cyclic AMP, which activates protein kinase A to phosphorylate certain proteins. During capacitation, there is an increase in membrane-bound phospholipase C, and this binding is highly stimulated by the addition of epidermal growth factor to the cells. The capacitated spermatozoon binds to the zona pellucida of the egg via specific receptors and it is suggested that the zona pellucida binds to at least two different receptors in the sperm head plasma membrane. One is a G(i)-coupled receptor that can activate phospholipase Cβ1 and may regulate adenylyl cyclase to further increase cyclic AMP concentrations. The cyclic AMP activates protein kinase A to open a calcium channel in the outer acrosomal membrane, resulting in a relatively small increase in cytosolic calcium. This increase in Ca2+ leads to activation of phospholipase Cγ, which is coupled to the second tyrosine kinase receptor. The products of phosphatidyl-inositol bisphosphate hydrolysis by phospholipase C, diacylglycerol and inositol-trisphosphate, induce the activation of protein kinase C and a calcium channel in the outer acrosomal membrane, respectively. Protein kinase C opens a calcium channel in the plasma membrane and, together with the inositol-trisphosphate-activated calcium channel, leads to a second and higher increase in cytosolic calcium. In addition, the depletion of calcium in the acrosome activates a capacitative calcium entry mechanism in the plasma membrane, leading to a rapid increase in cytosolic calcium (300-500 nmol l-1). This increase in intracellular calcium concentration (and pH) leads to membrane fusion and the acrosome reaction.