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Injection of mouse testes with aspermatogenic substance (AS) isolated from bull seminal vesicle fluid inhibited sperm production and caused a decrease in the weight of the testes. Subcutaneous injections of AS also caused a decrease in testis weight, but spermatogenesis was only slightly affected.Administration of AS (2 to 2·5 mg) to female mice 1 to 13 days after mating resulted in a highly significant decrease in the number of embryos. When concentrations of 12·5 μg AS/ml, or more, were present in the medium in which two-blastomere mouse embryos were incubated, further development was arrested in 100% of the embryos. Invivo, however, death of the embryos occurred only after nidation. Injection of AS into prepubertal males or females did not provoke any visible disturbances at the time, or later when sexual maturity was reached.The presence of AS in the diluent had no effect on the sperm motility or sperm survival time of bull, ram, boar and rabbit spermatozoa or on the fructolysis of bull and ram spermatozoa.Samples of AS showed no proteolytic, DNase, esterase, alkaline phosphatase, acid phosphatase or lactate dehydrogenase activities but had high RNase activity and AS was therefore named AS RNase. Incorporation of [2-14C]orotic acid into male mice injected with AS RNase made it possible to detect a significant reversible decrease of pyrimidine synthesis in testicular tissue. No decrease of RNA in liver and kidney tissue could be detected.