Does anyone know of a good technique to measure in vivo enzyme activity? For instance, I want to know if a degradative enzyme that can be found extracellularly in brain increases under certain behavioral conditions. Suggestions/link/etc greatly appreciated!

I think microdialysis is the technique you are looking for.

Here are some examples of its applications

Zidovudine Concentration in Brain Extracellular Fluid Measured by Microdialysis: Steady-State and Transient Results in Rhesus Monkey

Extracellular Brain Glucose Levels Reflect Local Neuronal Activity: A Microdialysis Study in Awake, Freely Moving Rats

Effects of salicylate on serotoninergic activities in rat inferior colliculus and auditory cortex

Acetylcholine Measurement of Cerebrospinal Fluid by In Vivo Microdialysis in Freely Moving Rats

Here's the wikipedia entry on Microdialysis

Microdialysis equipment

Can you tell us what the substrate for the enzyme is? perhaps there are analogues that will give you a readout in the blood or after you grind up some brain tissue?

Rb