This study was carried out to clarify the physiological effects of swimming training and dietary iron intake on rats. Fifty-seven male 15 weeks old rats were divided into three groups of different diets: low (L),medium (M) or high (H) iron diet. They were fed with the diets and water ad libtum during the experiment. After 4.5 weeks, rats in each diet group were divided into two sub-groups: control (Cr) and trained (Tr) rats. Cr rats were subjected to swimming for 10 minutes 3 days per week for 5 weeks in order to be familiarized with swimming, while Tr rats were trained with swimming for 60 minutes 5 days per Week for 5 weeks. After 5 weeks, Cr and Tr rats were forced to swim until exhaustion for determining their endurance performance. The training tended to increase blood hemoglobin (Hb) concentration and relative muscle weight (muscle weight/body weight) of soleus in H and M groups except in L. The relative muscle weights of soleus of Cr and Tr groups and Hb concentration of Tr groups tended to be increased with higher iron content in the diet. There was, however, no significant difference in the endurance swimming performance of Cr or Tr group among three dietary groups. These results indiucated that the dietary iron intake with swimming training increased the relative muscle weight of soleus and Hb concentration in blood, but could not improve the swimming performance.