Various lines of evidence suggest that pre-eruptive degassing of basalts is not only important but may dominate the He flux from the mantle to the oceans and atmosphere. These include: (1) correlations between U/ 4He ratios and Mg# in volcanic suites, which suggest that U/ 4He increases as differentiation proceeds; (2) the observation that U/ 4He ratios in MORB average about a factor of 400 lower than those from Loihi Seamount, although 3He/ 4He ratios suggest that Loihi is derived from a source with a lower U/ 4He ratio than is appropriate for the MORB source; and (3) the 3He budget of the oceans which, as defined by Craig and Lupton [6], requires that MORB is on average 70–90% outgassed in He. These observations suggest, as noted in [1], that mantle 3He/ 4He ratios may be subject to perturbation due to radiogenic accumulation of 4He in systems where U/ 4He has been increased by pre-eruptive degassing. Models involving continuous diffusive loss (CDL) of He from a magma chamber and exhalative loss (EL) via solution of He in a CO 2-rich gas or fluid phase are investigated. Low U/ 4He ratios in MORBs preclude significant pre-eruptive reduction of 3He/ 4He ratios; however, OIBs with U/ 4He > 10 6 may be subject to significant reduction of 3He/ 4He (⩾ 10%) with pre-eruptive aging over time periods ranging from 1 × 10 3 to 5 × 10 5 years.