PLA2 enzymes catalyze the hydrolysis of membrane phospholipids at the sn-2 position to release lysophospholipids and free fatty acids including the essential fatty acid arachidonic acid (AA). The biomedical role of PLA2 enzymes may be attributed to AA production playing an essential role in inflammatory signalling, or through other cell signalling events controlled by the enzyme. AA plays a vital role in normal cellular homeostasis and in a range of signalling and developmental functions including monoaminergic neurotransmission, brain development and synaptic plasticity, and its metabolites are also important in inflammation responses. Biomedical conditions which have been associated with altered PLA2 activity include cancer, ischemia, trauma and psychiatric disorders such as schizophrenia and bipolar disorder (manic depression). PLA2-catalysed overproduction of AA has also been shown in the brain following seizures, resulting in over-activated neurotransmission. The role of the PLA2 signalling pathway in these disorders remains unclear. A currently used treatment for epilepsy and bipolar disorder, the neuroprotective valproic acid (VPA), reduces the activity of the PLA2 signalling pathway in the brain by an unknown means. To further understand the role of this drug, we have investigated the cellular targets of VPA using the simple model system Dictyostelium. We have screened a mutant library for VPA-resistant colonies, and identified one ablated protein homologous to a poorly characterised human iPLA2 enzyme. Ablation of this protein gives rise to partial VPA resistance in Dictyostelium. We have also developed an in vivo assay for PLA2, and have used this to show that VPA inhibits PLA2-catalysed AA release in Dictyostelium. Using this assay, we have identified the well-defined structural requirements for PLA2 inhibition. These studies suggest that this VPA-catalysed inhibition may be related to the neuroprotective role of the drug.