Measurement of the plasma concentrations of oestradiol and progesterone in six bitches on 180 consecutive days showed large fluctuations in the levels of both hormones. The concentrations of oestradiol began to increase 10-12 days before the peak levels of 38.1-89.7 pg/ml were reached, then declined again over the next 3-7 days to or below the minimum measurable value of 9 pg/ml. On the day after the maximum concentrations of oestradiol were recorded, plasma progesterone began to rise rapidly, reaching a plateau after approximately two weeks, then declining gradually after a further two weeks. At the height of the luteal phase, peak levels of 12.6-70.1 ng/ml were measured, although on some days values of less than 1 ng/ml were recorded. The time of occurrence of the initial rise in the progesterone concentration during oestrus presumably indicates that preovulatory luteinization had taken place. During anoestrus the basal concentration of progesterone was generally less than 1 ng/ml and that of oestradiol less than 9 pg/ml. The normal values derived from these observations are discussed with regard to the interpretation of oestradiol and progesterone concentrations in domestic pets under treatment in veterinary practice.