By means of small-angle neutron scattering (SANS) experiments the dependence on pressure of the mean aggregation number [N] of rodlike tetradecyldimethylaminoxide (TDMAO) micelles in D2O was determined for pressures up to 2000 bar, at temperatures of 280.8, 294.9, and 308.1 K and at a TDMAO concentration c = 118 mM L-1. By an analysis via the so-called ladder model, the dependence of the difference of the chemical potentials on pressure for a monomer being located in the end cap and in the cylindrical part of the spherocylindrical micelles, respectively, was extracted. This difference depends on concentration c, pressure P, and strongly on temperature T. For the two lower temperatures, pressure-induced phase transition was observed.