The development of rhythmic 6-sulfatoxymelatonin excretion in urine was studied in healthy full-term and premature infants during the first 12 months of life. There was little evidence of rhythmic 6-sulfatoxymelatonin excretion before 9 to 12 weeks of age in full-term infants. Over this period, excretion increased five to six times compared to the excretion at 6 weeks (08 +/- 103 vs. 2973 +/- 438 pmol/24 h) with the major proportion of the hormone metabolite being excreted between 0200-1000 h. At 24 weeks of age, total 6-sulfatoxymelatonin excretion was 25% of adult levels. Premature infants (51 +/- 4 days premature) had a delay in the appearance of rhythmic 6-sulfatoxymelatonin of approximately 9 weeks. Even after correcting for gestational age or length of time at home, the premature infants were found to have a 2-3 week delay in the development of 6-sulfatoxymelatonin rhythmicity compared to full-term infants. These results provide evidence that neural centers responsible for rhythm generation and/or the pineal gland fail to accelerate their development after premature delivery. This may be due to the environment the infants are exposed to during their stay in hospital, particularly the pattern and intensity of lighting.