In the autumn and winter of 1979 and the spring and summer of 1980 serum samples were taken from approximately 20 sows at between 40 and 90 days of gestation in each of seven commercial herds. In most of these herds progesterone concentrations were significantly lower in the autumn than in the other three seasons. Between June 1980 and June 1981 every pregnant sow in a further herd of 250 sows was sampled at 25 to 30 days and at 70 to 91 days of gestation. Seasonal differences in progesterone concentrations were again evident, with the concentrations rising from their lowest in August, September and October to a peak in March. Comparisons between the early and late pregnancy progesterone levels from sows sampled in this herd at different times of the year suggested that corpora lutea which were formed in the summer and early autumn were fully competent and responsive, but that their lower hormone production was possibly the result of reduced luteotrophic stimulation. These findings are pertinent to the pathophysiology of the autumn abortion syndrome and other seasonal reproductive problems in sows.