The authors have studied the toxicity of alkylating agents on the gonads in the light of three cases. The more frequent prescription of these products in chemotherapy when treating cases in Nephrology, Dermatology, Neurology and Ophthalmology as well as general systemic diseases, should make the doctor prescribing them look out for the risks that the reproductive function runs in their use. Although of course there is no valid discussion of the use of these drugs when life is at stake one should all the same, seeing how their efficacy is increasing and survival is becoming more likely, ask oneself about their bad effects on the gonads, and in particular when they are being used for less serious indications. The gonads are often attacked early and insidiously, the the effects on them often starting before the more spectacular secondary effects such as alopecia or neutropenia. A review of the literature brings to light the following : there is a more marked effect on the gonad when it is the testis than when it is the ovary, which seems to be progressively affected; the lesions become worse and increase according to the pubertal state of the male. As far as testicular damage is concerned, it seems debatable whether is is dose-related or related to the length of treatment, and the chances of recovery on stopping chemotherapy with alkylating agents seen to be overall poor. On the practical level the use of sperm banks may resolve some of the worries about later infertility. As far as women are concerned, the greater resistance of the ovary to the effects should make one careful to employ contraceptive measures which are necessary during and after treatment (for fear of teratogenicity).