Objective: To evaluate the short-term responsiveness of rotatory thoracic end-play assessment to spinal manipulation and, thereby, motion palpation construct validity.Design: Prospective, single-blind, randomized, controlled trial (randomized blocks design). Setting: Laboratory, Center for Technique Research. Participants: Sixty first-year chiropractic college student volunteers; seventy-three possible candidates were screened. Interventions: The treatment group received manual high-velocity, low-amplitude rotatory manipulation. The control group received no intervention to minimize nonspecific effects of sham treatment. Main outcome measures: End-play response, defined as the change from restricted to normal end play immediately after intervention. Responsiveness, defined as the percentage of the end-play response attributable to spinal manipulation: relative response attributable to the maneuver, RRAM = (treatment group response--control group response)/treatment group response. Results: Ten percent of the tests were positive for restriction of end play in left or right rotation from T3-T4 to T12-L1; the average rate was 2.1 restrictions per subject (SD = 1.4). End-play response was 60% in the treatment group, in contrast to the 37% response in the control group (z = 1.86, p = .04). More than a third of the response in the treatment group was attributable to spinal manipulation (RRAM = 39%). For one examiner, RRAM = 51%. Mild symptomatology did not affect responsiveness. Conclusions: The data suggest a moderate short-term responsiveness of rotatory thoracic end-play restriction to spinal manipulation, hence it has utility as a posttreatment evaluative test. This study was the first to use an external standard (manipulation) to demonstrate that segmental end-play restriction changes, hence end-play restriction itself, are detectable in human subjects with manual palpation by chiropractors. Further research is required to determine the generalizability of the study findings.