OBJECTIVES: Heart rate variability (HRV) is often measured during clinical and experimental cardiovascular reflex tests (CRT), as a reflection of cardiac autonomic modulation, despite limited characterization of the rapid responses that occur. Therefore, we evaluated the responsiveness of HRV indices in 20 healthy young adults (age, 27 +/- 6 y; mass, 76.9 +/- 16.8 kg; height, 1.79 +/- 0.12 m) during four separate established CRT. METHODS: These included the [I] orthostatic challenge, [II] isometric handgrip, [III] cold pressor and [IV] cold diving reflex tests. Electrocardiogram was recorded throughout, with HRV derived from RR intervals at rest and from each CRT. On a separate day, a subgroup of participants (n=9) completed the same protocol for a second time. RESULTS: The maximal slope of heart rate change (dTdt) was significantly different between all CRT, with the orthostatic challenge producing the fastest increase (2.56 +/- 0.48) and the cold pressor the fastest reduction (-1.93 +/- 0.68) in heart rate. Overall HRV, reflected by Poincare plot ratio (SD1:SD2), was significantly reduced during all CRT ([I], -0.41 +/- 0.12; [II], -0.19 +/- 0.05; [III], -0.36 +/- 0.12; [IV], -0.44 +/- 0.11; p<0.05) relative to baseline and this was reproducible in time-series. However, when HRV indices were correlated to mean-RR an exponential growth-like relationship was evident (R(2) ranging from: 0.52-0.62). CONCLUSIONS: These unique outcomes demonstrate that short-term alterations in HRV are evident during CRT, while indicating the importance of adjusting for, or at least reporting, underlying heart rate when interpreting such measures.