Log transformation of concentration

Dear NONMEM users, I am working on a PK model and using the log-transformed concentration data. I'v read some discussions in the NONMEM user group about the log-transformed concentration. But I am still not very clear about this. Could anybody give me a reason to do the transform on concentration? Also, I am curious that after the transform, will the fixed effect have the same meaning as that in the untransformed model? For example, theta1 is the clearance, after log-transform of concentration, would the estimation of theta1 still stands for the population clearance? To my simple thinking about the differential equation, d(lnc)/dt= (dc/dt)*(1/c). Therefore, a "c" will be multiplied to the right term of the orginal differential equation. I think the solution of that equation might be different from the original one. If it is different, how can I explain the theta1 in the log transformed model? Would anyone please give me some explainations or references? Thanks a lot! Chenguang