The Internal Revenue Service
has indicated that research training programs under the National Institutes of Health (NIH) National Research Service Awards (NRSA) program, as well as those similar to or patterned after the NRSA program, do not result in wages to training stipend recipients, and therefore, do not require income
or FICA tax withholding
However, these stipends may still constitute taxable income
to the fellow/trainee to the extent there are no offsetting educational expenses. (Tuition, books, fees and supplies are qualified educational expenses; room, board, living expenses and travel are not for this purpose.)
See http://www.phds.org/nrsa-taxes/ for an article on NRSA fellowship taxation
The payments you receive fall under the general rule for scholarships, fellowships and grants that the amount that is not used for tuition, books, fees and supplies is included as income, but is not subject to self employment tax.
I hope this is the information you need for taxation of NRSA fellowship stipends.