We describe a simple, rapid procedure for measuring porphyrins in urine. After acidification of a urine specimen with hydrochloric acid, the second derivative of the absorption spectrum is recorded in the region of the Soret band maximum. The amplitude of the deflection is linearly related to porphyrin concentration, and the point at which the second derivative minimum occurs provides an estimate of the ratio of uroporphyrin to coproporphyrin in the sample. Nonspecific interference is not significant and reproducibility is excellent. Porphyrin excretion was studied in 24-h urine collections from men and non-pregnant as well as pregnant women. Median daily porphyrin excretion (nmol) was: men, 65.0; non-pregnant women, 60.1; and third-trimester pregnant women, 175.0. Distribution of these data was significantly skewed; upper and lower 2.5 percentile limits are given.