The nifedipine gastrointestinal therapeutic system (GITS) produces relatively linear fractional absorption-time plots between 6 and 24 h and about 70% of the available dose is absorbed within 24 h in human beings. However, conventional single-phase in vitro dissolution tests with the GITS demonstrate pseudo zero-order release of nifedipine suspension between about 2 to 20 h and ≥90% of the labeled dose is released within 24 h. Using the two-phase dissolution test developed in part I, mean in vitro data obtained from 2 strengths of the nifedipine GITS were compared with mean in vivo data from a human bioequivalency study in anticipation of improved in vitro-in vivo correlations. For the in vivo data, deconvolution was performed using the Wagner-Nelson method (model-dependent, WN) and DeMonS which is a new method of numerical deconvolution (model-independent, DS). Calculated zero-order rates of drug absorption (0.95(WN)/1.03(DS) mg h −1 and 1.88(WN)/1.94(DS) mg h −1; 30 mg and 60 mg strengths, respectively) compared reasonably well with the calculated zero-order rates of drug transfer (0.96 and 2.02 mg h −1; 30 mg and 60 mg, respectively). These values, however, are considerably less than zero-order drug release rates obtained using single-phase dissolution methods or reported as the manufacturer's design specifications (1.7 and 3.4 mg h −1; 30 mg and 60 mg strengths, respectively). Hence, improved in vitro-in vivo correlations were demonstrated with the two-phase dissolution test, yielding 1:1 (Level A) correlations ( r 2>0.99 in all cases).