Diets or supplements high in n-3 and n-6 polyunsaturated fatty acids (PUFAs) have been shown to influence the timing of parturition. PUFAs are substrates for prostaglandin (PG) synthesis, and PGs play central roles in parturition. Hence, the effects of altering PUFA composition may be mediated through alterations in the type and relative quantities of PGs synthesised. Therefore, we have investigated the effects of a range of n-3 and n-6 PUFAs in vitro on PG synthesis by amnion cells of late gestation ewes. The n-6 PUFA, arachidonic acid (20:4, n-6), increased synthesis of two-series PGs. Degree of stimulation induced by the n-6 PUFAs was dependent on the position of the PUFA in the PG synthetic pathway, i.e. PG production of the two-series (principally prostaglandin E(2):PGE(2)) increased progressively with longer chain PUFAs. Effects of n-3 PUFAs on output of PGE(2) were more modest and variable. The two shorter chain n-3 PUFAs, α-linolenic acid (18:3, n-3) and stearidonic acid (18:4, n-3), induced a small but significant increase in PGE(2) output, while the longest chain n-3 PUFA docosahexaenoic acid (22:6, n-3) inhibited PGE(2) synthesis. Dihomo-γ-linolenic acid (20:3, n-6), the PUFA substrate for synthesis of one-series PGs, induced an increase in PGE(1) generation and a decrease in PGE(2) and PGE(3) outputs. Hence, we have demonstrated that PUFA supplementation of ovine amnion cells in vitro affects the type and quantity of PGs synthesised.