Prostaglandins Ej and Fja play major roles in the initiation and maintenance of parturition. There is evidence that they also prime the uterus prior to parturition. Diets high in n3 fatty acids have been reported to be associated with impaired parturition, whereas uterine infection by the intracellular pathogen Chlamydia psiltaci is associated with abortion and premature labour. In each case the course of disruption of normal parturition is unknown, however, impaired or excessive 2-series prostaglandin production has been postulated to play an important part. The effect of high dietary n3 and n6 fatty acid intake on uterine fatty acid composition and metabolism by desaturase, elongase, phospholipase and cydooxygenase enzymes was investigated. The effect of C. psittaci infection on 2-series prostaglandin production and its control was also studied. The uterine fatty acid content of rats maintained on diets high in n6 and n3 essential fatty acids (EFA) for various periods was analysed and compared with a control group fed a normal pelleted diet. Rapid changes in uterine n6 and n3 fatty acid content were observed after three weeks feeding. However, in all three diet groups conservation of arachidonic acid was observed, which was highest in rats fed the n6 fatty acid diet and lowest in rats fed the n3 fatty acid diet. The 20C and 22C EFA were incorporated into phospholipids to a greater extent than into neutral lipids. The distribution of EFA in the individual lipid classes in the three diet groups indicated selective release of arachidonic acid and eicosapentaenoic acid into the free fatty acid pool. Phosphatidylethanolamine arachidonic acid levels were more susceptible to changes in dietary fatty acid content than those of phosphatidycholine and phosphatidylinositol. Analysis of prostaglandins produced by uteri of rats on the three diets, by mass spec-trometry, demonstrated an inhibitory effect of the n3 fatty acids on total prostaglandin production, and the synthesis of the 3-series prostaglandins E and F was detected. Pregnant sheep experimentally infected with an ovine abortion strain of C. psittaci were found to prematurely release prostaglandin Ej (PGEj) into the amniotic and allantoic fluids. Impaired release of PGE3 into the utero-ovarian vein was also detected in infected sheep. The plasma oestradiol-17/3 also increased earlier than that of control sheep. The study detected competitive inhibition of uterine n6 EFA metabolism at the level of esterification, chain elongation, desaturation and cyclooxy-genase metabolism by the dietary n3 EFA. In infectious abortion, abnormalities in PGEj and oestradiol-170 were detected. The first evidence of 3-series prostaglandin production by uterine tissue is presented.