The influence of age and sex on weight variation in rats treated chronically with lithium chloride.

Weight increase is a common side-effect after chronic lithium salt treatment in man. This effect has not always been found in animal models using the rat. The possibility that the age and sex of animals might interfere with the results was studied, using groups of male and female rats, 2, 5, and 12 months old, injected intraperitoneally twice daily for 28 days with a solution of lithium chloride, or sodium chloride as control. The dose used was 1.5 meq./kg body weight (total daily dose of 3.0 meq./kg). In 2 month old animals, lithium induced an increase in weight significantly greater than that of the controls in the female group, while the weight of the male rats in this age group remained indistinguishable from the control rats. The 5 and 12 month old lithium treated female rats showed no differentiation in weight. Lithium treatment of the males in these two age groups caused a large number of deaths and, considering only those animals which survived, average weights were significantly less than those of control in the 5 month olds rats, and equal to control in the 12 month olds. It is concluded that age and sex have to be taken into consideration when studying the effects of chronic lithium treatment on the weight of rats.