Reaction time occupies a foundational position in Jung’s experimental depth psychology, serving as the primary quantitative instrument through which the presence, intensity, and topography of emotionally charged complexes are mapped. Across the Experimental Researches corpus, reaction time is never treated as a mere chronometric datum; it is consistently interpreted as a psychodynamic index — a temporal signature of unconscious interference. Jung establishes that prolonged reaction times, those exceeding the probable mean for a given subject, mark the sites where a feeling-toned complex has disrupted the smooth flow of associative response. The instrument — a one-fifth-second stopwatch — is deliberately modest, yet its yield is theoretically rich: differential times across stimulus-words reveal gender differences (male subjects faster than female), educational gradients (educated subjects faster than uneducated), and grammatical-categorical effects (abstract nouns generating the longest latencies, concrete nouns the shortest among most groups, with educated men as a telling exception). Beyond statistical averages, reaction time functions diagnostically — in criminal investigation, differential psychiatric diagnosis, and psychoanalytic exploration — and in tandem with galvanometric and pneumographic measures, it triangulates the somatic signature of the complex. The core theoretical claim is that extended reaction time reveals how long the psyche requires to detach itself from a conscious or unconscious preoccupation and attend to the new stimulus, making it one of depth psychology’s most precise experimental windows onto the unconscious.