SUMMARY: Oestrone (OE1), oestradiol (OE2) and progesterone were measured by radioimmunoassay in the same samples of peripheral plasma from pregnant and lactating hamsters. The levels of progesterone during most of pregnancy did not exceed the values found during the first 2 days of the oestrous cycle. However, on day 14 of pregnancy (2 days before parturition) the level of progesterone abruptly increased to 29 ng/ml and fell rapidly thereafter. Hysterectomy on day 12 of pregnancy resulted in baseline levels of progesterone ( < 2 ng/ml) on day 13 and no peak in progesterone comparable to the one observed on day 14 of normal gestation. Injecting progesterone on day 14 or later prolonged gestation and caused maternal and foetal death. Oestradiol concentration showed peaks on days 7 (67·3 pg/ml), 11 (117·6 pg/ml) and 14 (81·8 pg/ml) of pregnancy and dropped precipitously between days 14 and 16 of pregnancy. The highest values obtained on day 11 correlated with the maximal proliferation of antral follicles. The abrupt decline of oestrogen concentration at parturition is different from the pattern found in the mouse and rat but is consistent with the lack of post-partum ovulation in the hamster. Injecting 50–150 μg oestradiol cyclopentylpropionate between days 12 and 16 did not interfere with parturition by the morning of day 16. Oestrone was detectable throughout pregnancy; the levels were lower than for OE2. In general, the oestrogen patterns were parallel. During lactation, progesterone secretion remained relatively constant at about 10–11 ng/ml, presumably synthesized by the hypertrophied interstitium which is the dominant ovarian tissue. In most instances OE1 and OE2 were not detectable during lactation, consistent with the absence of antral follicles.