This study investigated whether kava or valerian could moderate the effects of psychological stress induced under laboratory conditions in a group of healthy volunteers. Fifty-four participants performed a standardized colour/word mental stress task on two occasions 1 week apart. Blood pressure (BP), heart rate (HR) and subjective ratings of pressure were assessed at rest and during the mental stress task. Following the first session (time 1 = T1), individuals took a standard dose of kava (n = 18), or valerian (n = 18) for 7 days, while the remainder acted as controls (n = 18). Differences in BP and HR from resting levels were calculated as reactions to the stress task at both time points. At the second session (time 2 = T2) there was a significant decrease in systolic BP responsivity in both the kava and valerian groups relative to T1, but there were no significant reductions in diastolic BP. Between T1 and T2, the HR reaction to mental stress was found to decline in the valerian group but not in the kava group. Individuals taking kava or valerian reported less pressure during the task at T2 relative to T1. There were no significant differences in BP, HR or subjective reports of pressure between T1 and T2 in the controls. Behavioural performance on the colour/word task did not change between the groups over the two time points. The results suggest that kava and valerian may be beneficial to health by reducing physiological reactivity during stressful situations.