In order to investigate the effect of dopamine on Li+ balance during pregnancy, pregnant rabbits at 26 days of gestation were employed with lmg/kg of bromocriptine intravenously once a day for 3 days. On a study day, after an hour of third dose of bromocriptine, rabbits were infused with isotonic LiCl solution at a rate of lm^/kg/min through jugular vein over an hour. During constant infusion of LiCl solution, arterial plasma and urine samples were cellected at 15, 30 and 60 minutes. Plasma and urine concentration of Li+ and osmole were measured and with the volumes of urine collected at each time interval, changes of urinary excretion rate of Li+ and osmole were calculated. Clearances of Li+, osmole and free water during the time course of experiment were analyzed and compared with those of control rabbits. The results obtained were as follows * 1) Before LiCl infusion, urine flow rate and osmolar excretion rate were greater in bromocriptine treated pregnant rabbits than in control rabbits. 2) On the contrary, water and osmolar diuretic responses to Li+ were lesser in bromocriptine treated pregnant rabbits than in control rabbits. 3) Bromocriptine treatment enhanced gradual rise in plasma Li+ concentration, but reduced urinary concentration and excretion rate of Li+ during entire course of LiCl infusion. 4) Significant decrease in Li+ clearance shown in bromocriptine treated pregnant rabbits accompanied a considerable reduction in osmolar and negative free water clearance. The above findings suggest following tentative conclusions. A) Enhanced effect of bromocriptine on water excretion during pregnancy is mediated by prolactin depletion rather than by alteration of thyroid hormone or antidiuretic hormone release. B) However, role of bromocriptine in reduced diuretic response to Li+ could not be explanied solely by prolactin depletion. It could be mediated by reduced glomerular filtration rate due to decreased thyroid hormone and remained activity of water and solute reabsorption by increased antidiuretic hormone. C) Therefore, elevation of plasma Li+ concentration shown in bromocriptine treated pregnant rabbits may be associated with the reduction in glomerular filtration and acceleration in renal tubular reabsorption of Li+ via the alteration of thyroid hormone and antidiuretic hormone.