1. [(60)Co]EDTA has been evaluated as an extracellular marker in guinea-pig taenia coli and rabbit myometrium.2. The complex was found to be stable, non-toxic, convenient to use and had the advantage of being readily measured without interfering with the analysis of water and electrolyte contents.3. In both smooth muscles [(60)Co]EDTA distributed rapidly into a volume equivalent to that available to [(14)C]sorbitol and slightly greater than that available to inulin.4. The optimum concentrations of carrier required to effectively saturate any adsorption sites were 0.1 mM-CoEDTA for taenia coli and 1.0 mM for myometrium.5. [(60)Co]EDTA was distributed into a volume equivalent to the total water content of rabbit tendon, indicating that it was not excluded from connective tissue water.6. About 5% of the [(60)Co]EDTA taken up by smooth muscle in 15 minutes exhibited slow efflux kinetics. This property was also exhibited by [(14)C]sorbitol and has been reported for other extracellular markers.7. The efflux of 95% of [(60)Co]EDTA followed bulk diffusion kinetics for both taenia coli and myometrium.8. The simultaneous efflux of (24)Na indicated that 1-2% exchanged slowly while 98-99% of the (24)Na exchange could be described by bulk diffusion kinetics, similar to [(60)Co]EDTA. About 15% of this (24)Na was contained outside of the [(60)Co]EDTA space.9. More detailed studies involving the analysis of simultaneous efflux of [(60)Co]EDTA and (24)Na may help to locate and characterize the properties of the excess rapidly exchanging sodium (12-15 m-equiv/kg. wet wt.) in smooth muscle.