The effects on the oestrous cycle of the cow of uterine distension caused by injecting a viscous gel-like substance into the uterus were investigated. Intra-uterine injection in the early luteal phase shortened the cycle length in thirteen of fourteen cases, giving a mean of 12·5 days (P <0·01). Treatments during the late luteal phase lengthened the cycle in five animals, giving a mean of 25·8 days (P <0·01). Cycles were of normal length in ten of twelve cases treated at post-oestrus, functional luteal phase or pro-oestrus. In twenty-three of twenty-five cycles in which the treatments were performed between early luteal phase and late luteal phase, the mean length from the treatment to ovulation was 9·3 ± 1·6 days, suggesting that intra-uterine treatment is effective in synchronizing the oestrous cycle and ovulation in cows except during pro-oestrus, oestrus and post-oestrus. Gel treatment produced a temporary endometritis associated with cloudy mucus and numerous leucocytes and epithelial cells in the vagina. The retention of gel in the uterus was longer in animals treated at late luteal phase than in those treated at early luteal phase. Of six animals which received the intra-uterine treatment during early luteal phase three became pregnant after artificial insemination.