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1. The diffusion of deuterated water (HDO) into and out of isolated guinea-pig taenia coli and frog sartorius was followed by means of a recording electronic microbalance.

2. A comparison was made of the rates of efflux of deuterated and tritiated (HTO) water using the microbalance and a standard tracer technique.

3. The time course of the exchange of HDO in both smooth and striated muscle was fitted to the sum of two exponential functions; however, it was not possible to correlate sizes of the intra- and extracellular spaces in the muscles with the sizes of the two compartments in model systems based on compartmental theory of tracer kinetics.

4. The efflux of HDO, which was measured by means of the microbalance under stagnant conditions, occurred at a rate approximately 3 times slower than the efflux of HTO which took place while muscles were agitated vigorously in their bathing media.

5. There was a marked deviation at short times from a double exponential time course during the efflux of HTO.

6. Values of the tracer water permeability, P_d, of smooth and striated muscle fibres, and the extracellular space, E, of the muscles were obtained using a computer to numerically evaluate the series solutions of the equations describing the permeation of labelled water in the muscles.

7. It was necessary to assume a value for the diffusion coefficient of the tracer in extracellular fluid in order to compute a unique fit to an exchange curve.

8. Values of P_d and E given by the analysis depended upon the method used for monitoring the exchange, and an interpretation of this dependence was made difficult by the variation in the thickness of unstirred layers on the surface of the muscles under the different experimental conditions.