The concentration of progesterone and oestradiol-17β in peripheral plasma was determined by radioimmunoassay on each day of pregnancy in the mouse. The progesterone concentration, low on days 1 and 2, correlated with the histological development of the corpora lutea. Progesterone levels during the first half of pregnancy rose to a peak on day 6 with a significant decrease ( P < 0·02) on day 7. Peripheral progesterone concentration during the second half of pregnancy rose on days 12–13 with a peak on days 14–17, and declined after day 17, 3 days before the onset of parturition.

Oestradiol-17β concentrations were high on day 1, declined precipitously to low levels on days 2 and 3 recovering slightly on day 4. The levels then fell and remained low from day 5 to days 15–16, when there was a significant increase in oestradiol-17β concentration which continued until day 20.