From the NLM Technical Bulletin:
The List of Serials Indexed for Online Users (LSIOU), 2011 edition, is now available in XML format. The 2011 edition contains 14,287 serial titles, including titles currently indexed for MEDLINE® as well as titles indexed over time which have ceased or changed titles, listed alphabetically by the journal title abbreviation.
Tailored lists of indexed journals may be generated from the NLM® Catalog database Limits page. While the 2011 XML version of the LSIOU is a snapshot in time, this Limits page can be used to get a “real time” list for the LSIOU; select the Journal Subset called “Journals currently or previously indexed in MEDLINE” limit. For a list of only the currently indexed MEDLINE journals, select the Journal Subset called “Journals currently indexed in MEDLINE.”
See Also: Journals Recently Accepted by NLM for Inclusion in MEDLINE (Updated three times per year (after October, February, and June).