Melatonin levels have been studied in venous blood sampled at different frequencies (0.5-, 2-, and 60-min intervals) form intact ewes. All samples were taken during the dark phase of either natural or artificial photoperiods. In one experiment samples were taken simultaneously from both jugular veins to investigate the possible effects of "streaming" on the levels measured. Plasma cortisol was measured to ascertain whether or not the frequent removal of blood activated the ACTH stress axis. Plasma melatonin levels showed considerable variation with peaks of up to 365 pg/ml on a baseline of between 30 and 60 pg/ml. There was consistent evidence of intermittent peaks, the frequency of which increased with an increase in sampling frequency. Plasma cortisol showed no correlation with either the frequency or the amplitude of the melatonin peaks. When plasma samples were taken from both jugular veins a similar melatonin pattern was seen in the samples from both sides, but samples taken from the left jugular vein invariably showed higher levels than those taken from the right vein. This may be due to differential vascular drainage of the pineal to the two sides.