METHODS: Myometrial strips obtained from virgin guinea pigs were hung in organ baths with a resting tension of 1.5 g and allowed to equilibrate (3) prior to addition of oxytocin (OT) to initiate contraction. The NO donors S-nitroso N-acetylpenicillamine (SNAP, 100 µM) and 3-morpholiosyndonimine (Sin-1, 100 µM) were added in the presence or absence of OT (1 µM). Tension was recorded electronically and force quantified as described previously (2). Intracellular calcium was measured in single human myometrial cells isolated by collagenase digestion of patient samples obtained under an approved protocol from pregnant women undergoing caesar- ian section at term. Isolated muscle cells were grown on modified 35 mm dishes of our own design with glass bottoms. The NO donors SNAP and Sin-1 were added to cells that were pre-incubated with the calcium indicator dye Fura-2 (3 µg/ml) for 90 min. Cells were washed to remove unincorporated dye and imaged with a Nikon inverted mi- croscope (40x Fluor-S objective) coupled to a Photon Technologies Delta Ram able to excite alternately at 340 nm and 380 nm at video rates. Emission at 510 nm was detected using a CCD camera and ra- tioed for 340 nm/380 nm excitation. An increase in the ratio of fluores- cence in imaged cells over baseline was presented as the average cellu- lar change within the region of interest and presented as a calcium tracing over time where 500 frames min -1 are presented.