Proteins released into medium used to culture, for 24h, endometrium removed from guinea-pigs on Days 7 and 15 of the oestrous cycle, from ovariectomised guinea-pigs treated with progesterone and/or oestradiol, and from Day-15 pregnant guinea-pigs were separated using affinity chromatography on Blue Sepharose (BS) CL-6B into proteins with (BS+ ve) and without (BS-ve) affinity for this gel resin. The turnovers of total protein released from the endometrium were not significantly different between Days 7 and 15 of the cycle. However, the turnover of BS-ve proteins was significantly greater on Day 15 than on Day 7 of the cycle. This change in emphasis away from BS+ ve proteins to BS-ve proteins from Day 7 to Day 15 of the cycle appears to be regulated by oestradiol. Significantly greater amounts of [³H]-leucine were incorporated into secreted proteins synthesised by Day-15 endometrium than by Day 7 endometrium, the difference being 2.1-fold. [³H]-Glucosamine was also incorporated into secreted proteins synthesised by Day-7 and Day-15 endometrium in culture. However, there was no significant difference between the amounts of glycosylated protein synthesised and secreted by Day-7 endometrium compared to Day-15 endometrium, The secretion of proteins, synthesised de novo, was low in ovariectomised guinea-pigs treated with progesterone, higher in ovariectomised guinea-pigs treated with oestradiol, and at a maximum in ovariectomised guinea-pigs which had been treated with progesterone and then oestradiol. The secretion of proteins, synthesised de novo, by Day-15 pregnant endometrium was also low.