The effect of niacin supplementation at two levels: 0.0 (control) and 10 mg/Kg body weight on growth performance, respiration rate, rectal and skin temperatures as well as some blood constituents were studied. A total of 10 suckling Friesian calves, 19+0.34 day's age were used. The addition of niacin started at 39 kg body weight and lasted till 120 kg body weight. Animals were divided into two equal groups and housed indoors (semi-open shed) during the period from December to February. Each group was examined for growth performance, respiration rate, rectal and skin temperatures as well as some blood constituents. A treatment by period interaction was detected for body weight (P<0.05) and average daily gain (P<0.01). However, for the 90-day period, niacin suckling calves (NC) were 11 % more body weight and gained 17% faster than control suckling calves (C). Respiration rate and rectal temperature were higher at 14:00 h than at 8:00 h and in-group (NC) than in-group (C) at either period. Calves fed niacin had significantly higher concentrations of serum total protein, total globulin and its fractions (alpha, beta and gamma). It was not surprised to notice that serum total globulin concentration was 45% (P<0.02) greater while gamma-globulin elevated by 67% (P<0.01) in niacin-fed calves. Also treated calves had lower concentration of serum albumin (p < 0.01). Fed niacin (group, NC) increased concentrations of serum glucose by 22% (P<0.01),
total cholesterol by 18% (P<0.05), triglycerides by 40% (P<0.01) and transferase enzymes (aspartic transferase, and alanine transferase) by 16% (P<0.02) and 22% (P<0.01), respectively while creatinine concentration decreased by 24 % (P<0.01). Concentrations of serum calcium, inorganic phosphorus, urea-nitrogen and total bilirubin did not differ between groups. The results cleared that the benefits of niacin supplementation to suckling Friesian calves were achieved under cool winter temperature. Fed niacin succeeded in improving growth performance and increasing the supply of energy enough to support thermostability in cold stressed calves.