A recently proposed sensitivity technique called differential sensitivity theory is applied to the neu-tronic/thermal-hydraulic fast reactor safety code MELT-IIIB. This application centers on the develop ment and solution of the appropriate adjoint and sensitivity equations, resulting in an adjoint version of the MELT code called MELTADJ. Proper inte gration of the forward MELT solution with the corresponding adjoint MELTADJ solution formally yields sensitivity information for all input parameters. Two transients in the Fast Flux Test Facility were investigated by performing input parameter sensi tivity analyses. Sensitivities obtained via MELTADJ are compared with those from MELT recalculations using perturbed input. These investigations indicate sufficiently good agreement between differential sensitivity theory and recalculation to validate the development of MELTADJ.