We have shown independently that dietary fiber and n-3 fatty acids can affect gut function. This study investigated the interactive effects of resistant starch (RS) (as high amylose maize starch [HAMS]) and tuna fish oil on ileal contractility. Four-week-old male Sprague Dawley rats were fed 4 diets that contained 100 g/kg fat as sunflower oil or tuna fish oil, with 10% fiber supplied as alpha -cellulose or HAMS for 6 weeks. Fish oil feeding led to higher ileal n-3 fatty acid levels (mainly as DHA) and higher agonist-induced maximal contractility with an RS effect noted for carbachol. HAMS-containing diets resulted in lower colonic pH and higher total short-chain fatty acids (SCFA), but not for butyrate with fish oil. Low prostanoid responses in young rats were enhanced by fish oil independent of RS. The order of muscarinic receptor subtype responses were different compared to older rats; fish oil feeding altered the sensitivity of the M(1) receptor subtype. Although little interactive effects were demonstrated, these data suggest developmental changes in ileal receptor systems with independent effects of RS and fish oil on some bowel properties in juvenile rats.