The Medical Subject Headings (MeSH) are now available over the World Wide Web. The 1997 MeSH is available on the Web in several formats. From the NLM Web site, (http://www.nlm.nih.gov) click on General Information, Organizational Structure, Medical Subject Headings Section, then Downloading. It is possible to: browse the tree structures; download a file containing all of MeSH in tree structure; or download all of MeSH in MARC format. The downloads require the compl etion of an online Memorandum of Understanding before a download can be done. No license or fee is required. In the future it is hoped that MeSH will be available in other formats as well. Comments and suggestions are welcomed; please direct your correspo ndence to Stuart Nelson, MD, Head, Medical Subject Headings, e-mail: nelson@nlm.nih.gov.