Influence of body size on lithium levels in rats

Lithium chloride was injected into rats by the intraperitoneal or intravenous route. The dose was proportional to body weight, in the conventional manner. Lithium levels in blood serum and organs were determined after 3-24 h. Within a given strain, large rats had higher levels than small rats. The size of the rats, and not their age, was the determining factor. The large rats had more adipose tissue than the small rats. Inasmuch as lithium distributes in body water, the excess fat in large rats reduces its volume of distribution, which may be responsible for raising the lithium levels in aqueous compartments, including serum. Male and female rats of equal body size developed equal lithium levels in serum.