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The accumulated and annual levels of inbreeding and their effects on growth traits of lambs were investigated using data collected over 11 years at the National Animal Production Research Institute Zaria. Inbreeding level averaged 1.313% per year, cumulating to 14.45% over the years. Average individuals inbreeding coefficients of offspring from sires used for more than one breeding period was 0.103% across the inbred individuals and 0.004% over all lambs. Regression coefficients of growth traits (everage values per year) on inbreeding levels by year were – 0.185kg ± 1.820g and -0.391kg for birth weight, preweaning average daily gain and weaning weight, respectively. All regression coefficients indicated that birth weight and weaning weight declined by 0.185kg and 0.391kg, respectively while preweaning average daily gain increased by 1.820g. Conscious efforts to reduce inbreeding level and its effects on growth traits will greatly lead to improved productivity