Concentrations of fatty acids (FA) in prostatic tissue of patients with either benign or malignant prostatic disease have previously been shown to be significantly different. In particular, there was a significant reduction in arachidonic acid (AA, C204n-6) and docosapentaenoic acid (DPA, C22:5n-6) concentrations in malignant prostatic tissue (PCa) phospholipids (PL). It was suggested that the decreased AA concentration in PCa may be due to its increased metabolism via the cyclooxygenase (CO) and/or lipoxygenase (LO) pathways to produce eicosanoids such as prostaglandins (PGs) and/or leucotrienes (LTs) rather than an impairment in desaturase activity in situ. The eicosanoid production in benign prostatic tissue (BPH) and PCa was determined using C3H]AA. The only eicosanoid produced in significant amounts by either tissue was PGEz and PCa converted radiolabelled AA to PGEz at an almost 10-fold higher rate than BPH. PGE2 production from ['HIM by PCa was investigated in the presence of oleic acid (OA, C18:In-9), eicosapentaenoic acid (EPA, C205n-3), docosahexaenoic acid (DHA, C226n-3), dihomo-gamma-linolenic acid (DGLA, C20:3n-6). eicosatetraynoic acid (ETYA) and ketoprofen (KPN) respectively. OA was found to be the most effective inhibitor of PGE2 production by PCa compared with DHA, EPA, ETYA and KPN, while DGLA was the least effective. Diacylglycerol (DAG) formation from labelled AA by PCa was about 4-fold greater than in BPH. Such high levels of DAG may be a means of promoting tumorigenesis through activation of protein kinase C as found with phorbol esters which can be regarded as DAG analogues.