Cycle-resolved measurements of in-cylinder air velocity have been made in a motored engine (600-r/min engine speed, 10.6:1 compression) having a high-squish reentrant-bowl piston. The velocity fluctuations have been analyzed both by low-pass/high-pass frequency filtering and by evaluation of non-stationary velocity autocorrelation functions. These two complementary analyses and general issues pertaining to their interpretation are carefully examined. The results show the existence (and substantial squish-driven intensification) of demonstrably random fluctuations whose characteristic frequencies are sufficiently high to affect mixing and combustion within individual engine cycles. There is also evidence for lower-frequency fluctuations which are organized or deterministic to some extent within individual cycles but which are not repeatably phased from cycle to cycle.