Objectives/Hypothesis: The objective was to demonstrate close appositions between serotonin-immunoreactive boutons and laryngeal constrictor (LCon) motoneurons in Sprague-Dawley rats. Study Design: Animal experimental. Methods: LCon motoneurons were identified functionally by their antidromic responses to stimulation of the recurrent laryngeal nerve and postinspiratory modulation and were filled by intracellular injection of biotin amide (n = 6). The medulla was sectioned and, using immunohistochemical analysis, examined by light microscopy. Results: Serotonin appositions were found on all 6 LCon motoneurons, with an average number of 17 ± 6 close appositions per neuron. Conclusion: In comparison with the authors' previous study of inspiratory laryngeal motoneurons, the number of serotonin close appositions with LCon motoneurons was similar to that found with posterior cricoarytenoid motoneurons, but significantly less than that found with cricothyroid motoneurons. This finding may represent a basis for differences in tonic activity of laryngeal muscles observed in relation to the sleep-wake cycle.