We have used single-cell imaging to investigate intracellular Ca2+ signalling in human spermatozoa stimulated with progesterone (3 microM). In approx. 9% of cells progesterone caused the activation of slow repetitive [Ca2+]i (intracellular Ca2+ concentration) oscillations, with a period of 1-4 min, which persisted for the duration of recording (20-30 min). Pretreatment with nifedipine, which blocks T- and L-type voltage-operated Ca2+ channels in spermatogenic cells, did not modify the characteristics of the oscillations, but reduced the proportion of cells in which they were observed. Stimulation with Bay K 8644 or FPL64176 induced [Ca2+]i oscillations in 5-10% of cells, but their frequency was low (period, 4-5 min). Application of valinomycin (1 microM) to clamp membrane potential at E(K) (equilibrium potential for potassium) did not modify activity in oscillating cells, showing that plasma membrane potential and activation of voltage-operated conductances are not involved in the mechanism by which sperm [Ca2+]i oscillations are generated.