Reduction of data from the three Mariner 10 encounters to this date have allowed identification of helium and hydrogen as atmospheric constituents. Subsolar point densities are estimated at 4500 cm-3 for He and 8 cm-3 for the thermal component of H. A non-thermal component in H with a scale height of ~70 km has been observed near the limb off the subsolar point, providing a total apparent number density of 90 cm-3. Upper limits on other atmospheric constituents have been reduced an order of magnitude. A very tentative identification of O has been obtained but the uncertainty is large and we require further data reduction for confirmation.