The temporal patterns of synthesis of prostaglandin (PG)E₂ and PGI₂ by organ‐cultured fetal thymic lobes and the cell population(s) responsible for synthesis of such products within the murine fetal thymus have been investigated. Embryonic day 14 thymic lobes were organ‐cultured in defined media for 14 days and the media were collected every 24 h and replaced with fresh media. Collected media were processed for quantitation of either PGE₂ or PGI₂. Lobes were also cultured in 2′‐deoxyguanosine (1.35 mmol/L) to produce an enriched non‐lymphoid population. The per cent cyclooxygenase‐positive cells within non‐lymphoid cell‐enriched lobes as well as the capacity of such lobes to synthesize either PGE₂ or PGI₂ were determined and compared with that of intact thymic lobes. Results demonstrate that fetal thymic lobes, in vitro, differentially synthesize PGI₂ and synthesize PGE₂ at a constant rate. Moreover, lobes enriched for non‐lymphoid cells contain a greater percentage of cyclooxygenase‐positive cells and synthesize increased amounts of eicosanoids per 10⁴ cells compared with controls.