Purpose: To compare the dynamics of maximal oxygen uptake (V˙O2), blood lactate ([La]b), total energy expenditure (Etot), and contributions of the aerobic (Eaer), alactic anaerobic (Ean,al), and lactic anaerobic (Ean,lac) metabolic energy pathways over 4 consecutive 25-m laps (L0–25, L25–50, etc) of a 100-m maximal freestyle swim. Methods: Elite swimmers comprising 26 juniors (age = 16 [1] y) and 23 seniors (age = 24 [5] y) performed 100 m at maximal speed and then 3 trials (25, 50, and 75 m) at the same pace as that of the 100 m. [La]b was collected, and V˙O2 was measured 20 s postexercise. Results: The estimated energetic contributions for the 100-m trial are presented as mean (SD): Eaer, 51% (8%); Ean,al, 18% (2%); Ean,lac, 31% (9%). V˙O2 increased from L0–25 to L25–50 (mean = 3.5 L·min−1; 90% confidence interval [CI], 3.4–3.7 L·min−1 to mean = 4.2 L·min−1; 90% CI, 4.0–4.3 L·min−1) and then stabilized in the 2nd 50 m (mean = 4.1 L·min−1; 90% CI, 3.9–4.3 L·min−1 to mean = 4.2 L·min−1; 90% CI, 4.0–4.4 L·min−1). Etot (juniors, 138 [18] kJ; seniors, 168 [26] kJ), Ean,al (juniors, 27 [3] kJ; seniors, 30 [3] kJ), and Ean,lac (juniors, 38 [12] kJ; seniors, 62 [24] kJ) were 11–58% higher in seniors. Faster swimmers (n = 26) had higher V˙O2 (4.6  L⋅min−1, 90% CI 4.4–4.8 L·min−1 L·min−1, 90% CI 3.6–4.2 L·min−1), and Eaer power was associated with fast performances (P < .001). Conclusion: Faster swimmers were characterized by higher V˙O2 and less time to reach the highest V˙O2 at ∼50 m of the 100-m swim. Anaerobic qualities become more important with age.