Cyclic guanosine monophosphate (cGMP) has been shown to be important for the generation of vasomotion in rat mesenteric small arteries. The evidence for the importance of cGMP comes from the observations that removal of the endothelium prevents vasomotion (Gustafsson et al. 1993), prevents synchronisation of the Ca²⁺ activity in the vascular smooth muscle cells (SMCs) (Peng et al. 2001), prevents the depolarisation induced by Ca²⁺ release (Peng et al. 2001), and that all three functions can be restored by adding of 100-300 μM 8Br-cGMP to the bath solution. In the present study, we have tested whether intermediate concentrations of cGMP partly synchronise SMCs and whether cGMP modifies Ca²⁺ release from SR and hence vasomotion frequency. Wistar rats (n=33) were humanely killed and the mesentery was removed. Segments of small arteries were mounted for isometric (force) or isobaric (diameter) measurements after removal of the endothelium; spatially and temporally resolved Ca²⁺ was measured with confocal microscopy in the same preparations after loading with Calcium Green-1 AM. After removal of endothelium, no oscillations in tension were seen as reported previously (Peng et al. 2001). At intermediate (10-30 μM) 8Br-cGMP concentrations, a) beating of the oscillatory tension was seen, caused by oscillations with different frequencies in different parts of the artery (n=5), and b) Ca²⁺ imaging revealed islands of cells where Ca²⁺ transients were synchronised within an island but not between islands (7 out of 11 preparations). With 300 μM 8Br-cGMP, coordinated [Ca²⁺]_i activity and vasomotion were seen. Thus with increasing cGMP concentration, progressively increasing areas of synchronicity were seen, until at 300 μM 8Br-cGMP nearly the entire segment was synchronised. 8Br-cGMP concentration-dependently decreased vasomotion frequency (n=9) and the frequency of unsynchronised [Ca²⁺]_i waves (n=8) in hyperpolarised SMCs. Thus cGMP progressively reduced the propensity of the intracellular stores to release calcium. We conclude that cGMP has two effects on vasomotion: (a) to enhance intercellular coupling within the vascular wall, promoting vasomotion, and (b) to reduce the frequency of intracellular calcium release, inhibiting vasomotion. These two antagonistic effects may explain the different effects of cGMP in different vascular beds.