Concentrations of progesterone and luteinizing hormone were determined in peripheral blood plasma during the oestrus cycle of three non-lactating Afrikaner cows. Blood samples were drawn daily during the luteal phase (Day 3-16) and every 8 hours from Day l7 to 2 days after oestrus. Progesterone was measured by radioimmunoassay and LH by a double antibody radioimmunoassay. Progesterone levels increased gradually from Day 3, reaching maximum values of 6,3-13,3 ng/ml on Day l6-17. A temporary decrease in the progesterone concentration was found between Day ll and 14 of the cycle. Progesterone levels dropped from peak values to less than 1ng/ml within 48 hours and were followed by oestrus 50,7 hours later . Progesterone concentrations were lowest at oestrus (<1 ng/ml) but a small, short lasting peak was observed shortly thereafter. As this peak occurred l8- 25 hours after the pre-ovulatory LH surge it might coincide with time of ovulation. Base line LH levels averaged between 3,9 and 8,1 ng/ml for different cows. LH concentration increased as soon as the progesterone level dropped to less than 1 ng/ml. Peak LH levels (27,5-45,0 ng/ml) observed during the latter half of oestrus, were detected at a single sampling only and the release appeared to last less than 8 hours.