OBJECTIVES: To evaluate the effects of a foot bath on autonomic nervous system (ANS) activities as measured by pupil light reflex (PLR) and heart rate variability (HRV) in healthy adults. METHODS: PLR and HRV were assessed before and during a foot bath in healthy subjects (n = 110). The subjects dipped their feet in 40 degrees C water at a level below their knees for 15 min The light reflex in PLR was elicited at four ascending stimulus intensities. Linear-mixed effect models were fit with PLR parameters, and a Wilcoxon signed-rank test with HRV parameters. In PLR analysis, an interaction term between the foot bath intervention and the stimulus intensity was included as a fixed effect. RESULTS: The PLR analysis showed significant interactions in five out of seven parameters. The altered patterns indicated enhanced activity of the parasympathetic nervous system (PNS) due to the intervention. In contrast, the HRV analysis showed few alterations of PNS activities due to the intervention. CONCLUSIONS: The study results demonstrated differences in altered PNS activity during a foot bath, when assessed by PLR and HRV. In PLR, the stimulus intensity was a crucial factor for the evaluation of foot bath effects. Taken together, the combination of PLR and HRV effectively evaluates ANS activities during a foot bath.