Circadian rhythms of urinary prostaglandins: PGE2, PGF2 alpha and 6-keto-PGF1 alpha excretion were estimated by radioimmunoassay in ten healthy women aged 22-47. The correlation of prostaglandin excretion with cortisol, sodium and potassium levels was evaluated. All estimated prostaglandins had a characteristic rhythm of urinary excretion with higher levels during the day time than at night. Thus the day-night rhythm of each prostaglandin may depend on the same metabolic factors which regulate their renal synthesis. This may be important in the proper interpretation of prostaglandin behaviour in different pathological conditions.