Many American commenta­tors have noted differences in the use of these two words. The usual observation is that meantime nor­mally functions as a noun ("In the meantime,...") and meanwhile as an adverb ("Meanwhile,..."). The use of meantime as an adverb and meanwhile as a noun is gen­erally discouraged, although most commentators allow that such usage is not incorrect; one of them (Bernstein 1971) defends the adverb meantime at some length, not­ing its frequent use by Shakespeare.

The evidence shows that meantime and meanwhile have been used interchangeably as nouns since the 14th century and as adverbs since the 16th century. The gen­eral observation that meantime is now the more com­mon noun and meanwhile the more common adverb is undoubtedly true, but the adverb meantime and the noun meanwhile have been in continuous use for hundreds of years, and their use in current English is not rare:

Meantime I've had a letter from Paulhan and have written him —Archibald MacLeish, letter, 8 July 1949

Meantime there was a core of older contributors — Times Literary Supp., 19 Feb. 1971

Meantime, he is headed in the right direction —E. B. White, letter, 4 Feb. 1974

Meantime, ... those now in American racing have successfully cultivated a reserved, dignified image — Clive Gammon, Sports Illustrated, 1 Dec. 1986

And in the meanwhile, mum's the word —Alex­ander Woollcott, letter, 19 Nov. 1934

... were being developed in the meanwhile by engi­neers —S. I. Hayakawa, ETC, Summer 1952

But in the meanwhile a lot of people learned to read —Bergen Evans, "The Language We Speak," speech, June 1968

... gained a lot of other things in the meanwhile — People, 27 Apr. 1981

There is no need to make a point of avoiding such usage.(资料出处：韦伯斯特英语用法词典)