In this study, four PLGA microsphere formulations of Olanzapine were characterized on the basis of theirin vitrobehavior at 37°C, using a dialysis based method, with the goal of obtaining an IVIVC.In vivoprofiles were determined by deconvolution (Nelson-Wagner method) and using fractional AUC. Thein vitroandin vivorelease profiles exhibited the same rank order of drug release. Further,in vivoprofiles obtained with both approaches were nearly superimposable, suggesting that fractional AUC could be used as an alternative to the Nelson-Wagner method. A comparison of drug release profiles for the four formulations revealed that thein vitroprofile lagged slightly behindin vivorelease, but the results were not statistically significant (P<0.0001). Using the four formulations that exhibited different release rates, a Level A IVIVC was established using the deconvolution and fractional AUC approaches. A nearly 1 : 1 correlation (R2>0.96) betweenin vitrorelease andin vivomeasurements confirmed the excellent relationship betweenin vitrodrug release and the amount of drug absorbedin vivo. The results of this study suggest that proper selection of anin vitromethod will greatly aid in establishing a Level A IVIVC for long acting injectables.