Pineal gland size and serum melatonin levels in male rats are not correlated

As both the size of the pineal gland and the serum levels of melatonin are known to be variable it was decided to assess whether the two parameters are interrelated. For this purpose male Sprague-Dawley rats were killed at 12.00 and 24.00 h, respectively. Trunk blood was collected for melatonin radioimmunoassay. The pineal glands were prepared for histological examination, serially sectioned, and from planimetric tracings the volume of the gland was computed. Neither at day nor at nighttime was there a correlation between the volume of the pineal gland and the levels of serum melatonin. There was also no correlation between body weight and the volume of the pineal gland or the levels of serum melatonin.