Objective: In our study, the simultaneous determination of fluoxetine (FLX) and olanzapine (OLZ) was performed by absorbance subtraction and absorbance correction spectrophotometric methods. Material and Method: The active substances were determined by choosing the isosbestic point of 232 nm in the absorbance subtraction method and 255 nm and 245 nm wavelengths in the absorbance correction method. The accuracy of the methods was determined by applying the percentage recovery studies to the laboratory mixtures. The percent recovery values were found in the range of 98.1-100.2 for OLZ and 96.8-105.3 for FLX. The concentration range studied was 3.12-15.62 and 3.45-17.28 µg/ml for OLZ and FLX, respectively. Result and Discussion: In the study, two active substances used in antidepressant treatment were determined simultaneously. Today, these active substances used in the treatment have started to be used in combination in order to achieve a better effect of the treatment. Therefore, simultaneous analysis of two active substances becomes important. Two different spectrophotometric methods were used for analysis. The methods have been successfully applied and validated for the simultaneous determination of antidepressant active substances. Since the applied methods do not require pre-separation and can be applied directly, the amount of waste generated is reduced. Environmentally sensitive methods have been applied.