This study was designed to investigate the effect of early protein undernutrition of rats on later susceptibility to lead poisoning. Weanling rats, weighing about 55 grams, were malnourished by feeding low protein diets ( 12% and 5% casein diet ) during the four weeks of protein deprivation period. For the following 5 weeks, the malnourished rats were fed with 25% casein diet as rehabilitation diet. After the rehabilitation period, all rats were fed with 25% casein diet and the drinking water containing 2000ppm-Pb during the four weeks. The results obtained were as follows ; 1 ) Feed intake, body weight gain and food efficiency ratio were reduced in all rats exposed to lead, especially in rats fed with 5% casein diet during the four weeks of weanling period. 2 ) Enlargement of kidney and spleen were observed in all rats exposed to lead and were more remarkable in rats fed with 5% casein diet. 3 ) In rats exposed to lead, activity of serum glutamic oxaloacetic transaminase ( S- GOT ) was significantly increased by increasing the degree of early protein deprivation, whereas hematocrit value was significantly decreased. Activity of serum glutamic pyruvic transaminase ( S- GPT ) was shown a tendency to increase by increasing the degree of early protein deprivation, but the difference was not significant. 4) In rats exposed to lead, the amount of lead accumulation in kidney was increased by increasing the degree of early protein deprivation. The significant increase was shown in rats fed with 5% casein diet during the four weeks of weanling period. The results of the experiment suggest that early protein undernutrition, even after some period of rehabilitation, may enhance the later susceptibility to lead poisoning.