This study investigated the hypothesis that adaptations would occur in the sarcoplasmic reticulum in vastus lateralis soon after the onset of aerobic-based training consistent with reduced Ca²⁺-cycling potential. Tissue samples were extracted prior to (0 days) and following 3 and 6 days of cycling performed for 2 h at 60%-65% of peak aerobic power (VO₂(peak)) in untrained males (VO₂(peak) = 47 ± 2.3 mL·kg⁻¹·min⁻¹; mean ± SE, n = 6) and assessed for changes (nmol·mg protein⁻¹·min⁻¹) in maximal Ca²⁺-ATPase activity (V(max)), Ca²⁺-uptake, and Ca²⁺-release (phase 1 and phase 2) as well as the sarcoplasmic (endoplasmic) reticulum Ca²⁺-ATPase (SERCA) isoforms. Training resulted in reductions (p < 0.05) in SERCA1a at 6 days (-14%) but not at 3 days. For SERCA2a, reductions (p < 0.05) were also noted only at 6 days (-7%). For V(max), depressions (p < 0.05) were found at 6 days (172 ± 11) but not at 3 days (176 ± 13; p < 0.10) compared with 0 days (192 ± 11). These changes were accompanied by a lower (p < 0.05) Ca²⁺-uptake at both 3 days (-39%) and 6 days (-48%). A similar pattern was found for phase 1 Ca²⁺-release with reductions (p < 0.05) of 37% observed at 6 days and 23% (p = 0.21) at 3 days of training, respectively. In a related study using the same training protocol and participant characteristics, microphotometric determinations of V(max) indicated reductions (p < 0.05) in type I at 3 days (-27%) and at 6 days (-34%) and in type IIA fibres at 6 days (-17%). It is concluded that in response to aerobic-based training, sarcoplasmic reticulum Ca²⁺-cycling potential is reduced by adaptations that occur soon after training onset.