IDO1 and depressive disorder: <h4>Objective</h4>This study aimed to investigate changes in mRNA expression of the kynurenine pathway (KP) enzymes <i>tryptophan 2, 3-dioxygenase</i> (<i>TDO</i>), <i>indoleamine 2, 3-dioxygenase 1</i> and 2 (<i>IDO1</i>, <i>IDO2</i>), <i>kynurenine aminotransferase 1</i> and 2 (<i>KAT1, KAT2</i>), <i>kynurenine monooxygenase</i> (<i>KMO</i>) and <i>kynureninase</i> (<i>KYNU</i>) in medicated patients with depression (<i>n</i> = 74) compared to age- and sex-matched healthy controls (<i>n</i> = 55) and in patients with depression after electroconvulsive therapy (ECT).