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The replies in the recent postings may be strictly accurate, but I think perhaps in some cases the problem may be more complicated. In particular, the concentration of levodopa (as in Sinemet) in the blood may be measured as a function of time, but the concentration of dopamine produced from levodopa, within the brain, is not so easy to measure. First, the concept of a half-life implies the *assumption* that the decay in question follows a specific mathematical law. Second, I've felt for some time (and I think some experts agree) that the amount of dopamine in the brain has two components: There is a long-term, or "baseline" part that is independent of medication and reflects only the progression of PD; then there is the part that is *added* by taking levodopa as medication. I think it is only this second part that obeys the "half-life" rule of decay. Instead of that number, what I'd like to see is something that indicates how much of any given pill you need to take, in order to maintain a desired baseline level (that is, the level you would find in a subject without PD symptoms). Cheers, Joe -- J. R. Bruman (818) 789-3694 3527 Cody Road Sherman Oaks, CA 91403-5013