The authors studied the effect of pretreatment with internal use of lithium salts on analgesic activity of aminopyrine by hot-plate test in mice. The male mice at 4 weeks of age weighting 17-19 g were divided into 7 groups. Earch group being composed of 9-10 mice was given oraBy 0.1 ml of LiCI-solution per 10 g of body weight for 35 days, LiCl was dissolved in 0.9% saline solution, in such concentrations that each group received 0 mg, 50 mg, 100mg, 200 mg and 400mg of LiCl for each kg of body weight of mouse, respectively, Control group received only 0.9% saline solution. On the 1st, 3rd, 7th, 14th, 21st, 28th and 35th days of this experiment, 0.1 ml of 1% solution of aminopyrine per 109 of body weight was injected intraperitoneaJly and then tested 30 minutes later. In the hot-plate test, a mouse is placed on a hot plate being adjusted with the thermostat at 55°±1℃, and the response is licking or lifting the hind feet or an outright attempt to escape from the plate, The increase in response time is used to quantify the analgesic effect of aminopyrine, The results were as follows : 1. The response time in groups receiving 0-50 mg of LiCl per kg of body weight had decreased on the 14 th day, but it tended to return to the initial level after the 21st day of experiment. 2. The response time in groups receiving 100-400 mg of LiCl per kg of body weight had also decreased on the 14 th day, and the degree of the decrease in response time was greater than that of the former and then remained faily constant at this level to the end of the observation period. These results would indicate that LiCl inhibits the analgesic activity of aminopyrine.