RESUMEN
Hippocrates was the first physician to use the scientificn method to find rational and not religious or mythic causes, for the etiology of diseases. Hippocrates and Aristoteles did not dare to dissect the human body. Afterwards however, many scientists such as Heriphilus, Erasitrastus, Vesalus and Fallopio, performed experiments in human beings using vivisection. According to that age's ideas, there was no cruelty in performing vivisection in criminals, since useful knowledge for the progress of medicine and relief of diseases was obtained. Only during the 19th. century and with Claude Bernard (1865), the ethical principles of systematic scientific research in humans were defined. These principles were violated by nazi physicians during Hitler's dictatorship in Germany (1933-1945). As a response to these horrors, the Ethical Codes of Nuremberg (1947) and Geneva (1948), that restablished all the strength of Hippocratic principles, were dictated. The Nuremberg rules enact that a research subject must give a voluntary consent, that the experiment must be necessary and exempt of death risk, that the research must be qualified and that the experiment must be discontinued if there is a risk for the subject. The Geneva statement is a modernized hippocratic oath that protects patient's life above all. These classical rules, in force at the present time, are the essential guides that must be applied by physicians and researchers