Changes in heart rate variability (HRV) at "respiratory" frequencies (0.15-0.5 Hz) may result from changes in respiration rather than autonomic control. We now investigate if the differences in HRV power in the low-frequency (LF) band (0.05-0.15 Hz, HRV(LF)) can also be predicted by respiration variability, quantified by the fraction of tidal volume power in the LF (V(LF,n)). Three experimental protocols were considered: paced breathing, mental effort tasks, and a repeated attentional task. Significant intra- and interindividual correlations were found between changes in HRV(LF) and V(LF,n) despite all subjects having a respiratory frequency above the LF band. Respiratory parameters (respiratory period, tidal volume, and V(LF,n)) could predict up to 79% of HRV(LF) differences in some cases. This suggests that respiratory variability is another mechanism of HRV(LF) generation, which should be always monitored, assessed, and considered in the interpretation of HRV changes.