Calcium has an important role in the events of egg activation and early preimplantation development. We investigated changes in intracellular calcium concentration in human oocytes at fertilization using the calcium-sensitive photoprotein aequorin. Oocytes were donated for research by patients undergoing in-vitro fertilization treatment in the Department of Obstetrics and Gynaecology. Cumulus cells, and in some cases zonae pellucidae, were removed by appropriate enzyme treatment. Single oocytes were micro-injected with aequorin and incubated in a chamber perfused with pre-equilibrated culture medium in a photomultiplier system. Eleven zona-intact and 15 zona-free oocytes were incubated with sperm, and oocytes from each group were incubated without sperm as controls. Dramatic transient increases in intracellular free calcium concentration were recorded in three zona-intact and seven zona-free oocytes, thought to be the first direct measurements of intracellular changes in human oocytes at fertilization. The amplitude (up to 2.5 μM), duration (120 s) and frequency (every 10–35 min) of these transients were similar in zona-intact and zona-free oocytes. They resemble those recorded in mouse oocytes, which may therefore be a suitable model for biochemical events at human fertilization.