The acute toxicity tests lasting 96 hours were performed on Clarias gariepinus of mean weight 138±12g and mean total length of 28.0±1.5 cm. The LD50 of lead nitrate was 57.5 mg/l. The experiment lasted for 70days after the control and the experimental groups were exposed to sub-lethal concentrations (20mg/l and 35mg/l) of lead nitrate. Haemoglobin concentrations were significantly (p< 0.05) decreased to values between 6.64±0.23 g/dl to 5.42±0.58 g/dl in all the sub-lethal concentrations compared to the control values of 8.58±0.48 g/dl. Haematocrit values were similarly significantly (p< 0.05) lowered from the control value of 26.8±0.68 iu/l to 20.8±0.76 iu/l and 17.0±1.67 iu/l for 20mg/l and 35mg/l lead nitrates respectively. The mean values of RBC count were significantly (p< 0.05) lowered from the control value of 4.38× 1012±0.26 to between 3.39× 1012±0.12 and 2.72× 1012±0.02 in 20mg/l and 35mg/l lead nitrate respectively. The WBC count significantly (p< 0.05) increased from 20720.00±307.77 in the control to 20940±875.72 and 23620.00±402.01 in the two sub-lethal levels respectively. The serum enzymes of the control 40.00±4.07 iu/l (ALP), 34.8±1.18 iu/l (ALT) and 119.8±13.24 iu/l (AST) were significantly (p< 0.05) higher than the experimental group at 20mg/l lead nitrate 38.2±4.48 iu/l (ALP), 33.6±4.43 iu/l (ALT) and 113.4±14.77 iu/l (AST) while the values at 35mg/l lead nitrate were 31.0±5.84 iu/l, 26.2±2.67 iu/l and 97.6±14.15 iu/l respectively. There were no significant differences (p< 0.05) in MCHC, MCH and MCV values between the control and the experimental groups. This study has sufficiently contributed to the basic research needs of aquatic toxicology and fish pathology.