From the NLM Technical Bulletin:
he 2012 edition contains 14,417 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 2012 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.”
Journals newly selected for indexing are announced on this NLM Web resource page three times per year (after October, February, and June): Journals Recently Accepted by NLM for Inclusion in MEDLINE. Journals announced on this page are automatically part of the retrieval from the NLM Catalog searches.