This is a randomized, double-blind, placebo-controlled crossover study of single-dose intranasal oxytocin (24 IU) in 18-55 year-old men with attention deficit/hyperactivity disorder (ADHD). Following a screening visit to determine eligibility, participants will return for two main study visits. During the main study visits, study participants will receive either oxytocin (Syntocinon® nasal spray, Victoria Pharmacy, Zürich, Switzerland) or placebo (inactive ingredients of Syntocinon® nasal spray, Victoria Pharmacy), followed by assessments of cognitive control over attention and behavior. Twenty-four participants will be randomized 1:1 to one of two drug orders, i.e., oxytocin - placebo or placebo - oxytocin. In an additional neuroimaging substudy, a subset of participants will undergo task-based and resting-state functional magnetic resonance imaging (fMRI) following oxytocin/placebo administration to investigate the effects of oxytocin on fMRI activation and functional connectivity within the cognitive control network.