ForewordAlthough in the past the Swedish anatomical schools gave one of the first impulses to the study of biorhythms, this subject has never been a favourite matter of morphological investigations. The proposal of the organizing committee at the 8th International Anatomical Congress to organize a symposium on rhythmic research was willingly accepted. It was felt that as morphology is growing to be more and more an experimental discipline, biorhythms should be considered.It is gratefully acknowledged that investigators with great experience in biologic rhythms were prepared to take part in this symposium. Their different fields of research, physiology, biology, embryology and clinical, indicate the importance of rhythmic variations and mathematicians show how results gained have to be treated in a correct statistical fashion.There are the processes of cell multiplication, regeneration and growth, which have been proved to be rhythmic. The analyses of mitoses show to what extent such strong interventions as adrenalectomy, hypophysectomy etc. have been misinterpreted and delivered severally contradictory results when the rhythmic variations are not considered.The observation of tissues collected throughout 24 hours contributes to the much debated problem of "Amitosis" and also revealed a definite rhythm of DNA. In connection with this is the uptake and incorporation of DNA-precursors which occur rhythmically. In the vast number of precursor experiments which frequently vary strongly in their results, circadian changes have hardly been considered until now.It is not to be wondered at that biorhythmic oscillations are also expressed in the ultra-morphology, thus delivering pictures ranging from the "normal" of textbooks and atlases to those until now regarded as pathologic, or due to experimental influences, or technical mistakes.The group of histochemists finally communicate their results either by demonstrating the considerable size of variations in tissues and organs or by technical factors involved in the morphological analyses of rhythms. The latter brings new aspects in the histochemical technique and it seems that some hot discussions over enzyme localization, specifity, sex differences etc. resolves into a simple time factor, i. e. variations at the time of the day when tissues were removed for investigation.This symposium has given a wide view of the various aspects of biorhythms and it impresses the necessity for considering biorhythms in the interpretation of results in experimental morphology. The Springer Publishing Co., by publishing this Symposium, has made it possible to reach a much larger number of experimenters and we gratefully acknowledge our indebtedness to it.It is hoped that this symposium will serve as a guide, or at least as a sign post, in the fields of biological and medical research.