You could use FindMinimum to do the fitting. It's been in Mathematica since Version 1.

Thanks David,- great answer.

I use Mathematica to simulate pharmacokinetics and pharmacodynamics. These problems are described by a series of ODEs which NDSolve does a great job of solving - once I've chosen the values of the parameters.

Moving forward, I was wondering if I could use Mathematica (6.0) to take a data set and optimize the parameters in my model ODEs to give a best fit to the study data.

If this can be more easily done in the latest edition of Mathematica I am open to upgrading.

Thanks again for your time, Mark