In 35 young children, circadian rhythms of salivary cortisol levels were determined by ELISA using a commercially available kit with a minor modification. The concentration of labeled cortisol in the serum kit was reduced in order to measure cortisol in 10 μl of saliva. Intra- and interassay coefficients of variation for salivary cortisol ranged from 2.4 to 9.9 and 3.2 to 8.9%, respectively. Recovery of salivary cortisol was 82.9-107.0%. There was a highly significant correlation between cortisol levels in saliva and serum in adults (r = 0.857). Salivary cortisol levels ranged from 0.01 to 2.252 μg/l00 ml and showed significant diurnal variation in the children. Our ELISA is a precise, simple, noninvasive and useful method for clinical practice and study in infants and children.