To investigate (1) the effect of mental rehearsal (MR) on reaction time (RT), and (2) the roles of cerebral lateralization, memory, verbal and nonverbal processes in MR effects, two experiments were conducted using a total of 300 sports participants matched on age, cerebral lateralization, imagery ability, intelligence, kinaesthesis, motivation, sex, skill level and speed of reaction. It was found that (a) MR can affect significantly RT (p/0.01) because it is a powerful cognitive activity which can directly influence the memory system and make memory comparison and/or response selection processes more efficient, and (b) that there are two separate processing systems - two separate modes of thought - for verbally and nonverbally coded information, that these processing systems might be functionally discriminated along hemispheric lines, that the non-verbal processing system considerably precedes the verbal processing system, probably because it uses a more effective processing strategy, and that MR affects significantly (p/0.01) and differently the operations of the two processing systems, probably because MR is a cognitive activity more associated with the right cerebral hemisphere than with the left. The results are discussed in the light of current cerebral laterality, imagery, memory, MR and RT findings.