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A cat that has been neutered is infertile because his testicles have been removed. The exception to this would be if he had a retained testicle and only the one in the scrotum was removed, thus leaving the other to produce testosterone and sperm. The viability of the sperm is questionable as the temperature in the abdomen is much higher than in the scrotum and sperm develop better at lower temperatures but its not impossible. However, if he had 2 scrotal testicles and was neutered then he is not fertile any more. He may feel the desire to mount and engage in mating behaviors because other tissues can produce testosterone in small amounts but he will not be able to father babies.