Summary. Vaginal fluid was collected on tampons self-inserted for 4 h daily during the menstrual cycles of coitally active, coitally inactive, ovulatory and non-ovulatory women. The daily hourly rate of fluid collected and its concentrations of Na+, K+, Cl− and Ca2+ were assessed. The fluid revealed a characteristic composition throughout all the menstrual cycles of 'high potassium–low sodium concentrations' compared to plasma, but no characteristic changes with the stage of the cycle. Occasional transient increases and decreases in Na+ and Cl− concentrations were observed, usually closely correlated with one another. Transient changes in K+ also occurred but these were not always correlated with other ions. In 9 cycles from 7 subjects the mean ± s.e.m. levels (in mmol/kg fluid) of Na+ (82 ± 4), K+ (30 ± 2) and Cl− (79 ± 4) in the first 7 days of the cycles were not significantly different from the Na+ (75 ± 5), K+ (30 ± 2) and Cl− (72 ± 7) concentrations of the last 7 days of the cycles. This suggests that the differences in hormonal balance between the two phases of the menstrual cycle do not influence the ionic levels of vaginal fluid. The concentration of Ca2+ in 3 ovulating subjects varied between 1 and 5 mmol/kg fluid and often appeared to be inversely correlated with the amount of fluid collected.