To elucidate the mechanisms involved in the relaxing effect of 2-amino-4, 4alpha-dihydro-4alpha-7-dimethyl-3H-phenoxazine-3-one (Phx-1) on smooth muscle, we investigated its effects on the contraction of both intact and skinned (cell membrane permeabilized) preparations from the guinea pig taenia cecum. In intact preparations, Phx-1 concentration-dependently suppressed the contraction induced by either acetylcholine (ACh) or high-K(+) with an IC(50) value estimated at around 100 muM. Similar inhibitory actions of Phx-1 on force were observed in intracellular Ca store depleted preparations. In cell membrane depolarized preparations in the absence of extracellular Ca, however, Phx-1 had little effect on either caffeine- or ACh-induced contractions. In skinned preparations, Phx-1 suppressed Ca(2+)-induced contractions at concentrations higher than 100 muM. These results suggest that inhibition of smooth muscle contraction by Phx-1 is due mainly to inhibition of Ca(2+)-influx, although Phx-1 also seems to have direct inhibitory effects on the activities of the contractile apparatus.