SUMMARY: The circadian rhythm of urinary 17-hydroxycorticosteroid (17-OHCS) excretion in Europeans and Equatorial Amerindians has been compared. The precise daily habits of the Equatorial Amerindians did not result in a more marked rhythm of 17-OHCS excretion. Amerindian men and women excreted much less 17-OHCS than their European counterparts; the difference is still substantial when body weight is taken into consideration. The rhythm of 17-OHCS excretion in Amerindians had a minimum and maximum corresponding to their early reveille and bedtime confirming the importance of environmental stimuli in setting the timing of the rhythm.