Nocturnal serum melatonin profiles were determined twice for seven single women, during their time of employment as nurses (baseline), and after one year (follow up), in order to investigate the effects of shift-work on nocturnal melatonin secretion. All subjects were working in the same hospital under an irregularly rotating three-shift system. Five (5) mL blood samples were drawn six times at 2 h intervals between 20:00-06:00 hours under dim light conditions (< 50 lux). The same sampling procedures were repeated the following year. The results showed pronounced inter-individual differences in melatonin concentrations. There was a trend towards increasing maximum melatonin concentration (MAX melatonin) at follow up, with a similar tendency seen in summed melatonin (the sum of six measured melatonin concentrations). A trend was also seen towards increasing melatonin ratio at 06:00 hours (the percentage of melatonin concentration at 06:00 hours by summed melatonin) at follow up. Melatonin concentration at 06:00 hours was significantly higher at follow up, and a significant correlation between Morningness-Eveningness score (M-E score) at baseline and increased summed melatonin at follow up was also seen. These results suggest that: nocturnal melatonin secretion does not significantly increase after beginning shift-work; and that greater increases in melatonin secretion at follow up are found in subjects with higher M-E scones (increased morning type). With more subjects, however, there may be significant increase in MAX melatonin and/or summed melatonin in the follow-up study.