Prostaglandins (PGs) are naturally occuring, cyclic, unsaturated fatty acids which possess a wide range of potent biological activities. PGs have been found in human middle ear effusions and might have implications for understanding the inflammation and possibly the bone resorption seen in chronic otitis media. We have measured PGs by radioimmunoassay in middle ear effusions (MEE) from experimentally induced serous otitis media (SOM) and purulent otitis media (POM) in chinchillas. PGE2 levels were significantly higher in the POM group compared to the SOM group. We have also demonstrated that chinchilla middle ear mucosa can convert arachidonic acid (AA), a precursor of PGs, to PG by injecting 14C-AA into bullae and assaysing using radiochromatography. This conversion was completely blocked by both indomethacin and aspirin given orally or by direct injection into the middle ear. We then injected 50 μg of PGE2 into chinchilla bullae to assess its effect on the composition of MEE. First, the time course of PGE2 metabolism after its injection into the middle ear (ME) was determined by thin-layer chromatography (TLC) of labelled and unlabelled PGE2. Following this, serial daily injections of PGE2 and normal saline as control were made for one, three, and seven days. MEE and serum were collected as assayed for lactate dehydrogenase (LDH), acid and alkaline phophatase, calcium, protein and hexosamine. Compared to the control, the levels of LDH, acid and alkaline phosphatase, calcium and protein were significantly elevated. Hexosamine levels were higher than the control at one and three days but did not differ significantly at seven days from the control. We have therefore demonstrated that chinchilla middle ear mucosa has the ability to synthesize PG from AA and suggest an active role for PGs in the inflammation and in the bone resorption seen in otitis media.