Increased public interest in mindfulness has generated a burgeoning market in new consumer technologies. Two exploratory studies examined effects of InteraXon\'s "Muse" electroencephalography (EEG)-based neurofeedback device and mobile application on mindfulness-based relaxation activities. Psychophysiological outcomes (heart rate variability (HRV), electro-dermal activity (EDA), saliva amylase activity (sAA) and Muse application EEG "calm percent") were collected for two 7-minute conditions: Muse-assisted relaxation exercise (MARE), and unassisted relaxation exercise (URE). In the first study, participants (n = 99) performed both conditions in a randomized sequential design. A follow-up study used a randomized parallel condition (n = 44) to test for differences in HRV effects between the two conditions and extended follow-up observation. Generalized estimating equation models demonstrated a moderate increase in HRV following relaxation exercises, with no observable difference between MARE and URE conditions. Both MARE and URE conditions produced equally effective short-term increases in heart rate variability, without additional benefit from neurofeedback.