In the present study the pineal gland was examined in 2 outbred stocks and 6 
inbred strains of rats some of which were pigmented to varying degrees, to see 
whether inbreeding affects the variability and whether differences exist between 
albino and pigmented rats. The animals were kept under 12 h light: 12 h darkness 
(12 L:12 D) and killed 7 h after the onset of light and darkness, respectively. 
The parameters examined were pineal protein content, serotonin and melatonin 
levels and hydroxyindole-O-methyltransferase (HIOMT) activity. All the 
parameters examined revealed interstrain differences, independently of whether 
the data were expressed per pineal or per mg protein. The variation coefficients 
for the various parameters were relatively high. They were mostly smaller when 
the data were expressed per pineal rather than per mg protein. No striking 
differences existed between the variation coefficients in inbred and outbred 
rats. When pineal size and the melatonin-related parameters expressed per pineal 
were used to assess the melatonin-synthesizing capacity of the pineal glands, it 
was found that the outbred Wistar and Sprague-Dawley rats and the inbred 
LEWIS-derived (LEW/Han) rats, all of which were albinos, had the most active 
pineals. Intermediate activity was noted in the hooded E3/Han and BDE/Han and 
the albino BDII/Han rats. The smallest and least active pineals were found in 
the totally pigmented BN/Han and DA/Han rats. The results taken together show 
that different stocks and strains exhibit significant differences in pineal size 
and melatonin-forming capacity. Albino rats appear to have larger and more 
active pineals than pigmented rats.