5 inbred strains of rats maintained on 3 commercial pelleted diets were investigated for differences in haematological traits on the 154th day of age. Number, size and haemoglobin content of the erythrocytes were shown to be strongly influenced by strain and diet. While the strain AS/Ztm was always found in a high position within the ranking order of strains, the strain DA/Ztm was always found in a low position. For nearly all haematological traits, diet II (Altromin C1000) resulted in the highest values. This was not due to a difference in the content of iron (Fe), because even though diet III (Altromin 1534) contained only 50% of the Fe in diet I (Altromin 1324), differences between these two diets were small. Significant interactions between strain and diet were shown to exist for the mean corpuscular volume (MCV) and the mean corpuscular haemoglobin (MCH). Strong genetic correlations were found between haematocrit (HCT) and haemoglobin (Hb) and between MCV and MCH, while the number of red blood cells (RBC), Hb, HCT, MCV and MCH correlated significantly within each strain. However, these environmental correlations have to be interpreted with caution, because these variables are not determined independently.