Administration of lithium carbonate to sheep in the dose range of 600-1800 mg/day resulted in a linearly described rise in lithium levels in plasma and red blood cells. In contrast to the rodent model but in agreement with the human condition, plasma lithium levels exceeded those of red blood cells. Polydipsia and body weight changes were not evident. At plasma levels of 0.70-0.8 mM/l, lithium attenuated the ability of the pupil to constrict in response to 30-sec pulses of light in the 25-150 microW/cm2 intensity range but not the ability to dilate in the dark. Thus, similar to observations in normal human volunteers and bipolar patients, lithium reduces sensitivity to light. The sheep is proposed as a useful model for studying the actions of lithium.