An updated analysis and interpretation are presented of the magnetic field observations obtained during the Mariner 10 encounter with the planet Mercury on March 29, 1974. The combination of data relating to position of the detached bow shock wave and magnetopause and the geometry and magnitude of the magnetic field within the magnetospherelike region surrounding Mercury lead to the conclusion that an internal planetary field exists with dipole moment approximately 5.1¿1022 G cm3. The limited data set precludes quantitative determination of an intrinsic field more complex than a centered dipole. The dipole axis has a polarity sense similar to that of earth and is tilted 7¿ from the normal to Mercury's orbital plane. The magnetic field observations reveal a significant distortion of the modest Hermean field (350 &ggr; at the equator) by the solar wind flow and the formation of a magnetic tail and neutral sheet which begins close to the planet on the night side. Presently, an active dynamo mechanism in the planetary interior appears to be favored in the interpretation of the field origin, although fossil remanent magnetization cannot be excluded. The composite data set is not consistent with a complex induction process driven by the solar wind flow.