Dispersed day-17 fetal rat hypothalamic cells have been enriched according to size by velocity sedimentation prior to culture, and the SRIF production by these enriched populations was compared with that of other enriched cell fractions and with mixed-cell cultures. Cultures of mixed cells produced 100–400 pg SRIF/106 cells/4 h over a period of 28 days. Total SRIF production by mixed cells was inversely proportional to seeding density over the range 0.25–1 × 106 cells/ml/well and SRIF 14 and 28 were secreted in a ratio of approximately 6:1. Although secretory rates by low cell densities remained higher than those by high cell densities, SRIF production decreased with time at all seeding densities (up to 21 days). Dispersed fetal hypothalamic cells were enriched according to size by allowing them to sediment over 4 h through a shallow gradient of BSA in culture medium and subsequent cell fractions developed widely differing morphologies in monolayer cultures. In contrast to mixed-cell cultures, SRIF production at 8 days by both large and small cells were directly proportional to initial seeding density. Furthermore, the smaller cells secreted very much less SRIF than the larger cells (100 pg/106 cells/4 h vs. 1,300 pg/106 cells/4 h), whereas there was little difference in overall SRIF content (350 pg/106 cells vs. 400 pg/106 cells). Characterisation by HPLC of the SRIF content and secretion of smaller cells revealed SRIF 14 to 28 ratios of 7:1 and 3:1, respectively. In contrast in the large cells, the ratio was 1:1 for both content and secretion. Therefore, these cell groups contain and secrete different proportions of these 2 molecular forms of SRIF. In conclusion, we have demonstrated the existence of SRIF-producing cells which differ widely in relation to secretory rates and molar ratios of SRIF 14 and SRIF 28. This is unrelated to the seeding density of the cells and may reflect the existence of inherently different populations of somatostatin-producing cells in the hypothal