Incidence of upper extremity tendinopathies increases with exposure to forceful repetitive motion. Increased presence of neurochemicals has been observed in patients with tennis elbow. Young adult female Sprague-Dawley rats were used to examine the neurochemical response to repetitive forceful work tasks in forelimb flexor and elbow tendon tissues. Eighteen rats performed a high repetition high force task (HRHF; 60% maximum grip) in which grasping a lever occurred at a target rate of 4 reaches/min. Eight rats performed a low repetition low force task (LRLF; <15% maximum grip) at a target rate of 2 reaches/min. These tasks were performed 2 hrs/day, 3 dys/wk for up to 12 wks. Ten rats were controls. To examine for increased neurochemical production and their localization in distal flexor tendons, animals were euthanized with Nembutal, tissues collected, fixed in paraformaldehyde, and frozen-sectioned prior to immunohistochemistry using antibodies against NMDAr1 (BD PharMingen), SP (Chemicon) and CGRP (Chemicon). A microscope-linked bioquantification computer program was used to determine mean area fraction of neurochemical immunoreactivity (IR) in flexor endotenon, epitenon, and paratenon, bilaterally. Four-way ANOVA (group, week, limb, region) was used to determine differences. To examine for level of neurochemical production at the elbow, distal humerus and attached tendons/muscles were collected from 12 week HRHF and control rats, homogenized, flash frozen and stored at-80°C. Enzymelinked immunosorbant assays (ELISA) were then performed for SP (MD Biosciences) and CGRP (Alpco Diagnostics). Two-way ANOVA (week and limb) was used to determine differences. SP-IR was significantly increased in flexor peritendon (epitendon+paratenon) at 3 and 12 weeks and endotenon at 3 weeks in HRHF rats. SP-IR was not increased in the LRLF flexor tendons, nor was NMDAr1-IR. However, NMDAr1-IR was significantly increased in flexor peritendon and endotenon at 6 weeks in HRHF rats. CGRP-IR was significantly different in flexor tendons regions, with peritendon> endotenon, but not between exposure groups. In elbow tissues, ELISA CGRP levels were decreased significantly in week 12 HRHF compared to controls, while SP was not significantly different. The inclusion of bone with the elbow tendon tissues may have diluted the neurochemicals to below detectable levels. Our findings demonstrate that SP and NMDAr1 immunoreactivity increases in distal flexor tendon as a consequence of performing highly repetitive and forceful tasks. The response is tissue (peritendon>endotenon) and duration dependent. Increases in neurochemicals may be linked to persistent pain associated with tendinopathies of the upper extremity. Grant support CDC-NIOSH(MB), NIAMS(AB), and AAHS(JF).