Parenteral iron abolishes the "early anaemia' in all mammals tested and increases packed cell volume (PCV) and haemoglobin concentration (Hb) significantly above adult levels. In the present study we examined the effect of parenteral iron upon serum immunoreactive erythropoietin (siEpo) concentration in young mice (age 6-24d) and in adult iron-deficient mice. Young BALB/CJ mice, from 5 to 23 d of age, and adult (60-100 d old)mice, some of which were iron deficient due to a low iron diet from the time of weaning, were given iron subcutaneously (iron sorbitol, Fe+3, 1.2 mg iron/100g body weight/injection). Iron was given as one single dose and the animals killed 20-24 h later. Control mice were similarly treated with saline. In young mice, from the age 10 d to weaning at 20 d, the siEpo level, measured 20-24 h after a single subcutaneous dose of iron, was 5 times greater than that in control mice (P < 0.0001). In contrast, there was no change in siEpo 20-24 h after a single subcutaneous dose of iron in adult mice, whether iron-deficient anaemic or normal. Thus, during the "early anaemia' in young mice over 10 d old, iron increased siEpo levels, whereas it had no such effect in iron-deficient and normal adult mice. This suggests that between ages 10 and 20 d, iron has additional effects in the regulation of erythropoiesis, which may not occur in adult or in 6-d-old mice.