Background: Daily practice of device-guided slow breathing (DSB) has been shown to decrease blood pressure (Chaddha et al., 2019). However, there is still a lack of understanding of the mechanisms underpinning the improvements in blood pressure. Relatively few studies have characterised the acute cardiovascular responses to DSB, which hold the key to the mechanisms by which DSB might lower blood pressure. This study characterised the acute cardiovascular responses to different DSB protocols. Methods: Eighteen healthy, normotensive women completed five 5-minute protocols in randomised order: spontaneous breathing (SfR), fixed breathing frequencies of 4, 6 and 8 breaths.min-1 (4fR, 6fR, 8fR) and a dynamic breathing frequency (DfR) determined by an algorithm designed to maximise respiratory sinus arrhythmia (RSA). Cardiovascular variables and respiratory airflow were monitored continuously and non-invasively. Data are mean±SD, compared by repeated measures ANOVA with planned pairwise comparisons. Results: Average breathing frequency for DfR was 6.3±1.1 breaths.min-1. Mean heart rate was not significantly different between breathing protocols, but RSA increased significantly between SfR and all DSB protocols (SfR 0.12±0.05, 4fR 0.25±0.10, 6fR 0.25±0.08, 8fR 0.21±0.07, DfR 0.25±0.09 sec; p<0.001). The ‘peak-valley’ amplitude of intra-breath (inhalation vs exhalation) fluctuations of mean arterial pressure (MAP) were significantly different between 4fR and SfR, 6fR, 8fR and DfR (SfR 5.1±1.87, 4fR 7.8±11.81, 6fR 6.8±10.81, 8fR 9.1±2.38, DfR 4.8±11.30 mmHg; p<0.001). Peak MAP occurred during expiration for all protocols except 4fR. Intra-breath fluctuations also increased during DSB for stroke volume, cardiac output, systolic and diastolic blood pressure, and pulse wave velocity (p<0.001). Conclusions: DSB induces significant increases in intra-breath fluctuations of haemodynamic variables. It is conceivable that these acute haemodynamic perturbations generate error signal(s) for chronic regulation of blood pressure. Further research is required to understand the acute responses to DSB in people who have hypertension.