Slow-paced breathing (SPB) is an accessible and non-invasive training that is consistently associated with promoting relaxation and reducing stress. The objective of the present study is to evaluate the effects of SPB on stress level measured with the salivary cortisol and the autonomic nervous system (ANS) measured through the heart rate variability (HRV). High school students (n=25, 8 male/17 female, 14-16 years old) were enrolled. RR intervals were measured using POLAR (V800 Monitor). The protocol consisted of 3 phases each lasting 5 minutes: baseline (REST), SPB and a recovery phase (POST). Saliva samples were collected after the REST and POST phases. The frequency of SPB was set to 6 cycles per minute (0.1Hz) and paced by means of a mobile application. The protocol #CAAE 49629221.8.0000.5505 was approved by the Institutional Research Ethics Committee of the Universidade Federal de São Paulo. Salivary cortisol was analyzed with ELISA kit (KGE008B, R&D Systems kit). RR intervals were subdivided into 2-minute subsequences for each phase of the protocol, filtered to remove ectopic beats and resampled in the time domain at 2 Hz. We computed the mean heart rate, the power spectral components and nonlinear indices, e.g. the sample entropy. The activation of vagal modulation during SPB persists in the recovery phase, as demonstrated by the decrease in cortisol, although modest, REST: 1.982 ± 0.075; POST: 1.951 ± 0.058 [ug/dl], p<0.01 paired t-test. The difference between POST and PRE was characterized by a positive correlation between cortisol and the LF/HF ratio of the HRV power spectrum (Spearman coefficient: 0.497, p<0.05). SPB has demonstrated significant potential as an effective strategy for stress management. Funding: FAPESP Process No. 2024/14706-7.