In their transit through the female genital tract, mammalian sperm acquire the ability to fertilize the egg in a process called capacitation. During this event the intracellular levels of cAMP and cGMP increase, suggesting that cyclic nucleotide-gated (CNG) channels, which have been identified in mammalian sperm, play a functional role in their physiology. Here we report an electrophysiological characterization of the effect of cyclic nucleotides on mouse sperm. Using the patch-clamp technique in the whole-cell configuration, we show that macroscopic ionic currents are augmented by the addition of both, 8Br-cAMP and 8Br-cGMP to non-capacitated mouse sperm. Although cyclic nucleotide regulates the activity of CNG channels, disparate effects of cyclic nucleotides may also occur. Addition of L-cis-diltiazem (50 microM), a specific inhibitor of CNG channels, partially blocked currents elicited by cGMP, suggesting that CNG channels play a role in the fertilization capability of mammalian sperm.