The contractile responses to 10(-6)M acetylcholine (Ach) and 100 mM K+ solution have been studied in the rat ileal smooth muscle exposed to calcium-free media (with or without 1.0 mM EGTA) to further examine the calcium pools mobilized by these agents for activation of contraction. Both stimulants produced contractions in Ca-free media, only in the absence of EGTA. The phasic component of the contractile responses to Ach as well as the tonic component of K+-induced contractions were relatively resistant to short periods of Ca-free exposure; this suggests that Ach mobilizes Ca from some tightly-bound or intracellular pool, whereas high-K+ contraction involves mobilization of Ca from some superficial or loosely-bound pool. Both Ach and high-K+ still caused phasic contractions following D600 treatment, with high-K+ stimulation also maintaining a tonic component, suggesting that in this tissue, high-K+ contractions are relatively resistant to calcium deficiency compared to Ach.