Contents Introduction Methods of assay Assay based on relaxation of the symphysis pubis : (a) guineapig, (b) mouse Assay based on inhibition of spontaneous contractions of uterine myometrium: (a) in vivo, (b) in vitro Electrometric assay Occurrence, source and concentration in different species Preparation, biochemistry and pharmacology Toxicity and antigenicity Effects Relaxation of pelvic joints Inhibition of uterine myometrial activity Reduction of tension in the cervix Effect on weight, water content and histological and biochemical composition of the uterus Role in pregnancy and parturition in the rat and mouse Effect on mammary gland Anaemia, antidiuresis and ureteral function Identification of the different activities Clinical tests Gynaecological and obstetric conditions : (a) dysmenorrhoea, (b) premature and term labour Scleroderma and peripheral vascular disorders Conclusions INTRODUCTION Relaxin is a name given to a hormone or hormone complex produced during pregnancy in the ovaries and/or reproductive tracts of females of many species. For