This thesis comprises two parts both dealing with the modulation of 5-lipoxygenase (5-LO), the enzyme responsible for the synthesis of pro-inflammatory leukotrienes (LT). First, the impact of pregnancy was studied. LT formation was higher in blood from pregnant compared to non-pregnant females. This higher LT synthesis in blood of pregnant females is influenced by synergistic and opposite effects: (I) higher numbers of LT forming cells (“plus”), (II) lower LT formation capacity of isolated granulocytes (“minus”) and (III) upregulating effects of plasma from pregnant donors on LT formation from isolated enzyme and cells (“plus”). These results suggest that LTs might be involved in the immune regulation during pregnancy. In the second part dealing with benzoquinones as exogenous modulators of 5-LO new inhibitors were identified and their molecular mode of inhibition characterized. The natural compound embelin acted as potent inhibitor of 5-LO and microsomal prostaglandin E2 synthase-1 (IC50 = 0.06 and 0.2 µM). New targets of embelin are presented for which lower concentrations are needed for inhibition than for others described before. Besides embelin, the benzoquinone RF-Id was studied in cell-free, cellular and blood assays for its interference with LT synthesis and identified as potent 5-LO inhibitor. Mechanistic studies showed that embelin and RF-Id, though similarly structured, interfered with 5-LO in different modes. Interestingly, both inhibitors do not inhibit via redox-type 5-LO inhibition as often supposed for benzoquinones in literature. After being activated by reduction in the cell RF-Id interacts with 5-LO in a nonredox-type fashion. Structure activity relationships of 31 embelin derived benzoquinones all modified in backbone position 3 with alkyl or prenyl chains revealed distinct features for potent inhibition of 5-LO. The most potent compounds were ortho-quinones (IC50 = 0.03 to 0.06 µM) which are valuable for future pharmacological studies.