OBJECTIVE: To determine whether colonic smooth muscle dysfunction is involved in the pathogenesis of idiopathic megacolon in cats. DESIGN: In vitro smooth muscle mechanical measurements. ANIMALS: Colon from healthy cats and cats with idiopathic megacolon. PROCEDURE: Colonic smooth muscle strips were suspended in physiologic buffer solution, attached to isometric force transducers, and contracted with acetylcholine (ACh; 10(-9) to 10(-4)M), substance P (SP; 10(-10) to 10(-6)M), cholecystokinin (CCK; 10(-11) to 10(-8)M), potassium chloride (KCl; 10 to 80 mM), or electrical field stimulation (EFS; 25 V, 1 to 30 Hz, 0.5-millisecond duration). Isometric stress responses were compared with those obtained from healthy controls. Colonic smooth muscle strips were also evaluated histologically for neuronal and smooth muscle cell morphology. RESULTS: Passive isometric stress was not altered, but the active isometric stress responses of megacolon smooth muscle to ACh, SP, CCK, KCl, and EFS were significantly (P < 0.05) diminished, compared with healthy controls. Differences were observed in longitudinal and circular smooth muscle from proximal and distal portions of the colon. Histologic evaluation revealed few abnormalities of smooth muscle cells or of myenteric or submucosal plexus neurons. The contractile response of megacolon smooth muscle to EFS, and the inhibition of this response by tetrodotoxin, suggest that myenteric and submucosal plexus neurons in megacolon smooth muscle are functional. CONCLUSIONS: Idiopathic megacolon is a generalized dysfunction of colonic smooth muscle in cats. The diminished isometric stress responses to receptor occupancy (ACh, SP, and CCK) and membrane depolarization (KCl) further suggest that the disorder involves disturbance in the activation of smooth muscle myofilaments.