Flow is the subjective experience of effortless attention, reduced self-awareness, and enjoyment that typicallyoccurs during optimal task performance. Previous studies have suggested that flow may be associated with anon-reciprocal coactivation of the sympathetic and parasympathetic systems and, on a cortical level, with astate of hypofrontality and implicit processing. Here, we test these hypotheses, using the computer gameTETRIS as model task. The participants (n= 77) played TETRIS under three conditions that differed in difficulty(Easy b Optimal b Difficult). Cardiac and respiratory activities, and the average oxygenation changes of theprefrontal cortexwere measured continuously with functional near infrared spectroscopy (fNIRS) during performance.The Optimal condition was characterized by the highest levels of state flow, positive affect, and effortlessattention. The associations between self-reported psychological flow and physiological measures were investigatedusing a series of repeated measures linear mixed model analyses. The results showed that higher flowwas associated with larger respiratory depth and lower LF. The higher respiratory depth during high flow isindicative of a more relaxed state with an increased parasympathetic activity, and thus provides partial supportfor the main hypotheses. There was no association between frontal cortical oxygenation and flow, even at liberalthresholds; i.e. we found no support that flow is related to a state of hypofrontality.