10mM sodium fluoride inhibited the output of prostaglandin (PG)F_2α from Day-7 guinea-pig endometrium and the outputs of PGF_2α and PGE₂ from Day-15 guinea-pig endometrium during 24h of tissue culture. The incorporation of [³H]-leucine into cellular and secreted proteins by Day-15 guinea-pig endometrium in culture was inhibited by over 90% of sodium fluoride (10mM). Thus, as far as endometrial PG and protein synthesis are concerned, the effects of sodium fluoride are similar to those of other protein synthesis inhibitors and provide further evidence that the increase in endometrial PGF_2α synthesis at the end of the oestrous cycle is dependent on the stimulation of fresh protein synthesis by oestradiol acting on a progesterone-primed uterus. Over a shorter period of culture (1h), sodium fluoride (10mM) stimulated the outputs of PGF_2α, 6-keto-PGF_1α and PGE₂ from Day-7 guinea-pig endometrium. However, the other G-protein modulators, cholera and pertussis toxin, had no effect on the output of PGs from Day-7 or Day-15 guinea-pig endometrium in culture for up to 24h. Consequently, a toxin-insensitive G-protein may be involved in mediating PG release from guinea-pig endometrium. Sodium fluoride also increased the outputs of PGF_2α, 6-keto-PGF_1α and PGE₂ from the Day-7 and Day-15 guinea-pig uterus superfused in vitro. The sodium fluoride-mediated increase in PG output from the superfused Day-7 uterus was unaffected by removal of extracellular calcium but was prevented by the intracellular calcium antagonist TMB-8. The calmodulin inhibitors, W-7 and trifluorperazine, and the phospholipase C inhibitor, neomycin, had no effect on the sodium fluoride-stimulated increases in PG output from the superfused Day-7 uterus. Therefore, sodium fluoride appears to stimulate uterine PG output by releasing calcium from intracellular stores, and this release is not due to activation of PLC nor is it mediated by calmodulin.