Faraday rotation measurements of polarized extragalactic sources probe the Galactic magnetized, ionized interstellar medium. Rotation measures of these sources behind the inner Galactic plane are used to explore characteristics of the structure in the spiral arms and in interarm regions. Structure in the spiral arms has a characteristic outer scale of a few parsecs only, whereas interarm regions typically show structure up to scales of hundreds of parsecs. The data indicate that in the spiral arms, the random component of the magnetic field dominates over the regular field, but in the interarm regions the random and regular field components may be comparable, and a few times weaker than the random magnetic field in the spiral arms.
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