Expansion of the cumulus mass results in dilution of cumulus cells in accumulated hyaluronic acid. This process is associated with an increase in progesterone secretion. The present study was carried out to evaluate the relationship between these two processes in vitro. Dilution of human cumulus cells was attempted by changing the cell-plating density of cumulus cells in culture. Concurrent with an increase in cell-plating density from 0.25 x 10(4) to 8 x 10(4), progesterone secretion increased by 10.5 times (P < 0.001), 9 times (P < 0.001), and 5.9 times (P < 0.001), and oestradiol secretion increased by 1.4 times (P < 0.001), 1.1 times (P > 0.05), and 2.6 times (P < 0.005) during days 0-3, 3-5, and 5-7 of culture, respectively. However, when steroid secretion was expressed in terms of ng per number of cells, the increase in cell-plating density from 0.25 x 10(4) to 8 x 10(4) coincided with a decrease in progesterone secretion of 3 times (P < 0.001), 3.5 times (P < 0.001), 30 times (P < 0.001), and 12.5 times (P < 0.001) during days 0-3, 3-5, and 5-7 of culture, respectively. The progesterone:oestradiol ratio increased gradually with the increase in cell-plating density. However, at the higher range of cell-plating density the progesterone:oestradiol ratio decreased with extension of the culture period. Addition of human FSH (hFSH) or hCG increased progesterone secretion when the cumulus cells were plated at 2 x 10(4), but in most cases not at 0.25 x 10(4) or 8 x 10(4) cells per dish.(ABSTRACT TRUNCATED AT 250 WORDS)