We detected a HPLC peak corresponding to 6-keto-PGF(1 alpha), a stable metabolite of prostacyclin (PGI(2)), in [1-C-14]arachidonate metabolites from the ciliated protozoan Tetrahymena thermophila. Quantitative analysis of 6-keto-PGF(1 alpha) by enzyme immunoassay revealed that the synthesis and release were rapidly activated by the mechanical stimulation of a short centrifugation. The activation was suppressed significantly by a cyclooxygenase inhibitor, ibuprofen, and was independent of the extracellular Ca2+. External addition of PGI(2) and its stable analogue, beraprost, caused a transient increase in the tumbling frequency of swimming. Other prostanoids, PGE(2) and PGF(2 alpha), have no effect on the swimming. These results indicate that a free-living ciliate, T. thermophila, synthesizes and has a specific sensitivity to PGI(2).