||These experiments were conducted to study the effects of Azadirachta indica admixed in poultry feed on weight gain performance, haemtological values,immune modulations, and toxic effects in broiler chickens. A total number of 144 commercial broiler 1-day old chicks were reared in the experimental sheds of the Department of Pathology, University of Veterinary & Animal Sciences, Lahore, The birds were fed with balanced commercial feed and water ad libitum. The birds were divided into 3 groups; A, B and C having fourty eight chicks each. Birds of all groups were sub divided into four groups of each i.e. A1, A2, A3 and A4; B1, B2, B3 & B4 and C1, C2, C3 and C4, respectively. Each of the sub groups containd 12 birds. Sub groups A4, B4 and C4 were control group with no medication. The birds of groups A, B and C were fed with poultry feed containing dry powder of neem leaves @ 2 gm, 4 gm and 6gm per kg of feed respectively. The birds of groups A1, B1 and C1 were treated with the herb from day 0 to 42 of their life. The birds of groups A2, B2 and C2 were given the neem from day 14 to 42 of their life, whereas the birds of groups A3, B3 and C3 were treated with the herb from day 28 to 42 of their life. Difference between weekly weight gain in the birds of groups A1, B1 and C1 was non significant (P>0.05) however the difference between weight gain in the treated and control groups was significant (P<0.05). The birds treated with the herb from day 0 of their life showed more weight gain. There was no difference in the haematological indices between all of the treated groups and the control groups. The neem treated birds showed increased antibody titers against ND and IBD viruses as compared to control groups. The values of ALP and ASTshowed decreasing trend when the level of neem leaf meal was increased in the ration. Serum creatinine and serum uric acid values posed a slight declining trend in the neem fed birds. There was a decrease in serum cholesterol level in the neem treated bird groups, the higher the concentreation of the herb, the lower the cholesterol value. The organ body weight indices showed that there was no significant difference in liver, spleen and thymus weights among treated groups as well between treated and control groups. There was absence of prominent gross pathological lesions in liver, spleen, kidneys and thymus, however some treated groups showed mild hypertrophied liver and kidneys as did the organs of the birds in control groups. No histopathological changes except a few mild changes were observed in liver, spleen, kidneys and thymus in the birds of experimental groups.