Blood was collected from the cerebral sinuses and from the jugular vein of 5 ewes during both the day and night. Cerebral sinus samples were collected by means of a permanently indwelling cannula (roughly every 5 min) while jugular vein samples were collected by venipuncture (roughly every 10 min). In each of the 5 animals mean nighttime melatonin concentrations were greater at night than during the day. In 2 animals, cerebral sinus plasma melatonin concentrations were greater than in the jugular vein; in 2 animals the sinus and jugular plasma had similar melatonin levels; in 1 ewe jugular vein blood melatonin levels exceeded those in the cerebral sinus plasma. These differences among animals are presumably due to slight positional differences in the cerebral venous cannula placement. In several animals episodic release of melatonin was apparent. Whereas the episodes were most obvious in the cerebral venous blood at night, they were also apparent in 1 case in the jugular vein plasma and in 1 animal during the day. When episodes appeared they occurred about every 15-20 min.