The hypothesis that urinary porphyrins could be excreted in a circadian rhythm was examined. A group of 20 healthy subjects, 10 males and 10 females, aged 20-40 years, were used in this study. Urine was collected at 4-hr intervals over three 24-hr periods. Median porphyrin concentrations (microgram l-1 or microgram g-1 of creatinine) were similar in the six groups, but significant differences were observed in that excreted. Highest porphyrin excretion was detected in the afternoon (12.00-16.00 hr) and, in the evening (16.00-20.00 hr). pH values were also increased at evening but did not correlate with porphyrin excretion. Despite these temporal variations, no cycles indicating circadian periodicity were observed. Therefore, a 24-hr collection of urine is not necessary for the initial evaluation of porphyrin metabolism.