In the boron-supplemented groups with adequate vitamin D3, both proximal and dis- tal tibial growth plates narrowed significantly. Although this might mean that the growth rates of the animals were limited, it might also show that the animals had healthy bones. There are only a few studies reporting a close relationship between boron and blood param- eters such as haematocrit (HCT) and haemoglo- bin (Hb) content and counts for leucocytes, erythrocytes and monocytes. In the present study, it was found that Hb and HCT amounts significantly increased in the 25 mg/kg boron- supplemented group. However, the elevation of plasma iron concentration was much greater than the increase in plasma Hb in all groups. Mean corpuscular volume (MCV) (data not shown) may be a better indicator than Hb or HCT values for plasma iron concentrations. Leucocyte counts, red blood cell counts, throm- bocyte counts and their peripheral blood per- centages were not affected by any level of boron supplementation. Increases in HCT and Hb values for the 25 mg/kg group may be related to elevated plasma iron content (expect from group 6) and copper concentrations obtained from the 5 and 25 mg/kg boron supplementation groups. However, it can be concluded that high Hb values are not necessarily beneficial in fast growing broiler chicks and may in fact be symptomatic of cardiopvascular stress. Also, increased resistance to blood flow through the lung can cause pulmonary hypertension in broilers (Crespo and Shivaprasad, 2003). Hunt (1989) reported that 3 mg/kg boron addition to cholecalciferol-deficient chicks had no effect on HCT or Hb values. However, Seaborn and Nielsen (1994) found that HCT and Hb concen- trations of male rats were significantly depressed by high dietary additions (500 mg/kg) of boron.