Progesterone has an extragenomic action on human spermatozoa characterised by the rapid induction of a calcium transient followed by a plateau phase during which [Ca2+], remains significantly above baseline. By imaging the calcium responses generated in individual cells, we have demonstrated that during this plateau phase, spermatozoa exhibit a series of asynchronous secondary calcium oscillations. The incidence of such oscillations was dependent upon sperm capacitation and showed significant inter-individual variation. The oscillations were dependent upon the influx of extracellular calcium via mechanisms that were insensitive to inhibitors of L-type voltage operated calcium channels (nifedipine, verapamil, diltiazem), G-proteins (pertussis toxin) or the GABA (A) receptor (bicuculline). However, treatment with an inhibitor of the GABA-associated chloride channel (picrotoxin) significantly suppressed the incidence of secondary calcium oscillations in pentoxifylline-treated cells, as did two inhibitors of T-type calcium channels (pimozide and amiloride). We hypothesise that the sub-population of spermatozoa exhibiting secondary calcium oscillations are characterised by a hyperpolarized plasma membrane that sets T-type channels in a closed but activation-competent state. The secondary calcium oscillations created via these channels do not induce acrosomal exocytosis per se but may prime the cells so that this event is rapidly triggered when the spermatozoa make contact with the zona pellucida.