This study was undertaken to observe the effect of neem oil, on the microscopic structure of testes in mature male albino rats and the associated changes in the serum levels of male reproductive hormones. The animals were divided in different groups as A₁ = treated males at low dose (0.6 mL of neem oil/animal), A₂ = treated males at high dose (1.2 mL of neem oil/animal), A₃ = controls for group A₁ (corresponding dose of peanut oil) and A₄ = controls for A₂ (corresponding dose of peanut oil). Animals were kept under observation for a period of six weeks. At the end of this period animals were anesthetized, blood was removed by cardiac puncture and sacrificed. Testes were removed and fixed in10% formolsaline for microscopy and methanol for HPLC purpose. The microscopic sections of the testes from animals treated at high dose have revealed significant decrease in the seminiferous tubular diameter and number of spermatozoa. Interstitial spaces are shown to be wide with possible decrease in the number of Leydig cells. These changes are associated with significant alteration in the serum levels of male reproductive hormones and presence of higher concentrations of active neem compounds in testicular tissue.