A chronobiological study was carried out in 10 elderly male subjects (78–83 years) to evaluate the 24-hour thyroid-stimulating hormone (TSH) secretory pattern. 10 young adult males (26–35 years) made up the control group. Hourly blood samples were drawn from each subject for a 24-hour period. TSH levels in elderly subjects showed blunted circadian fluctuations compared to those seen in young adult subjects. Mean 24-hour TSH values in elderly (3.1 ± 0.3 μU/ml) and young adult subjects (3.5 ± 0.1 μU/ml) did not differ statistically, but nighttime TSH values observed in elderly subjects (3.2 ± 0.3 μU/ml) were lower (p < 0.05) than those recorded in young adults (4.1 ±