Abstract: Eighty adult male Sprage-Dawley rats, assigned into groups (5 in each) and given one dose (50 ?L) of different concentrations of formalin solution (2.5, 5 and 10%) intratestically. Weight, food and water consumptions were monitered. Animals were maintained in a controlled atmosphere 21?C?1?C under 12 hrs light: 12 hrs darkness schedule. Paraformeldehyde, prilled 95% obtained from Aldrich Chemical Company (Milwaukee, WI 53233, USA) was dissolved in water and different concentrations were prepared. One and two months post injection animals were kept for ten days for mating with virgin untreated females to evaluate mating capability of treated rats. Animals were sacrificed and the following organs were collected for evaluation: testes, epididimidis, seminal vesicles, preputial glands, liver and kidneys. Testes and epididymidis were cut in halves, one half was placed in 10% neutral buffered formalin and processed following standard histological procedures. Formalin injection intratestically did not result in any apparent changes on condition or behavior of the treated rats. However, reduction in size and weights of the testes, epididymidis and seminal vesicles were obvious. Also, bilateral injection resulted in impairment of mating capabilities for all concentration used. These changes were less pronounced in unilateral groups. Sperm counts decreased in testes and epididymidis in all treated groups. Histological changes appeared to be more severe close to the locus of injection with the seminiferous tubules filled with connective tissue and sperm debris. The degree of changes beyond the central lesion showed poor structural status having one or two cell layers with no sperm inside the lumen or very few depending on the concentration of the formalin used. Intact and normal histological architecture were observed at the periphery of the testes and much larger number in the 2.5% group. Intertubular connective tissue thickened and cells were replaced with collagenous tissue especially in testes injected with 10% formalin solution. Epididymidis were devoid of sperms in their lumen. Besides, vacuoles were observed within the epithelium of the epididymis. No histological changes were observed within the liver and the kidneys of all groups. This study showed that a single dose intratesticular injection of formalin solution has as adverse effect on the reproductive system of the male rats. This experiment could be modified and used as a new method of chemical castration especially if it is used in young animals (before puberty).