The oral administration of a soxhlated crude ethanolic extract of leaves of neem (Azadirachta indica Ajuss; family Meliaceae) to adult male mice for 6 weeks (one spermatogenic duration) at the rate of 0.5, 1.0 or 2.0 g/kg body weight per day increased the incidences of structural changes and synaptic-disturbances in meiotic chromosomes and also caused more disruptions of meiosis. The extract reduced the sperm count and increased the frequency of spermatozoa with abnormal head morphology. It is suggested that at least one of the constituents of the extract may have interfered with the DNA. The result was chromosome strand breakages, or spindle disturbances, and the regulation of genes responsible for sperm shaping was affected.