The response of the brain to external stimuli (administration of fluoxetine) was monitored with in vivo SPME and MD as an established approach [6,73]. While MD was able to accomplish analysis of only two out of four targeted neurotransmitters, due to the low sensitivity of the method, SPME provided information for all selected substances, showing superior preconcentration abilities. However, it was also shown, during global analysis of the brain metabolome, that integration of both analytical approaches allows for extension to untargeted studies of lipids and other highly hydrophobic species, due to the better affinity of SPME-extraction phases towards non-polar compounds, and the higher affinity of MD to very polar compounds that are not extracted by the SPME fiber.