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In this article, reference images for raters, and sample images used for rater training are provided. All raters independently indicated that the RGS score is significantly elevated with this methodology in reserpine-induced fibromyalgia-like rats.•The grimace scale, a method for rating facial expression, is applied to the reserpine-induced fibromyalgia-like rat, which manifests nociplastic pain.•Facial expression change in the reserpine-induced fibromyalgia-like rat is substantial and long-lasting.•Elevation of the RGS score in the reserpine-induced fibrom