A micellar electrokinetic chromatographic method has been developed to analyze biological (human serum, saliva and urine) and environmental samples (three different water samples) for letrozole (LE), fluoxetine and their main metabolites. For this purpose a 20 mM borate buffer (pH 9.5) containing 20 mM SDS and 12% v:v 2-propanol was used as the background electrolyte. The samples were hydrodinamically injected for 6 s, separated in a fused-silica capillary at 25 kV and 50 degrees C and detected at 230 nm. Under these conditions, the migration times for all the studied compounds ranged from 3.0 up to 8.0 min. Linearity ranges were determined as 125-1500 ng/mL, whereas detection limits were from 37 to 120 ng/mL in biological samples and a value of 6 ng/mL in water samples. According to the validation study, the developed method was proved to be accurate, precise, sensitive, specific, rugged and robust. This method was applied to the analysis of different biological fluids at clinical levels, including two urine samples from patients undergoing treatment with LE or fluoxetine, and also to environmental samples at potentially polluting level. Prior to the determination, the samples were purified and pre-concentrated by means of an extraction-preconcentration step with a C-18 cartridge and by eluting the compounds with methanol.