Abstract: The highly toxic environmental contaminant, 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) was recently reported to decrease serum melatonin levels throughout the circadian cycle in the most TCDD-susceptible strain of rat, Long-Evans. To find out whether this effect is related to the mechanism of acute lethality of TCDD, serum melatonin levels were measured at the nocturnal peak phase in the most TCDD-resistant rat strain variant, Han/Wistar rats, 6 hr to 28 days after TCDD exposure. The same dose as used in the previous study, 50 μg/kg, decreased serum melatonin levels to approximately half the control values by the first day after the treatment. Melatonin concentrations remained at this reduced level over the whole observation period. In an auxiliary experiment, Han/Wistar rats were dosed with 1000 μg/kg TCDD and killed on day 3. Neither light nor electron microscopic examination of their pineal glands revealed any alteration attributable to TCDD treatment. These findings might indicate that the reduction of serum melatonin levels in the rat by TCDD is not related to its lethal effect and is not due to a direct damage of the pineal gland.