We demonstrate the room-temperature operation of a silicon nanoelectromechanical resonant-body field effect transistor (RB-FET) embedded into phase-locked loop (PLL). The very-high frequency resonator uses on-chip electrostatic actuation and transistor-based displacement detection. The heterodyne frequency down-conversion based on resistive FET mixing provides a loop feedback signal with high signal-to-noise ratio. We identify key parameters for PLL operation, and analyze the performance of the RB-FET at the system level. Used as resonant mass detector, the experimental frequency stability in the ppm-range translates into sub atto-gram (10(-18) g) sensitivity in high vacuum. The feedback and control system are generic and may be extended to other mechanical resonators with transistor properties, such as graphene membranes and carbon nanotubes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4758991]