To evaluate the accuracy of various techniques for determining the sexual state of sows, four multi-parous Landrace sows were housed in stalls in the Veterinary Hospital, Zurich University, 2-5 weeks after weaning. Catheters were inserted under anaesthesia into a prominent ear vein, and four days later blood and urine sampling commenced for 25 days, along with physiological and behavioural measurements (length, width and depth of the vulva, vulval colour, vaginal pH, presence and elasticity of vaginal mucus, rectal temperature, backpressure test, score for interest in each other, in their own or each others’ urine, the occurence of flehmen and the characteristic oestrous ear position. Blood serum was analysed by radioimmuno assay for oestradiol-17ß and progesterone. Urines were analysed for oestrone, oestradiol-17 ß, progesterone derivatives, creatinine and optical density.