Tanks a lot, it works!

but now I have a new question: there are two solutions for each of the variable of the system. Mapple gives me the solutions in the form of "rootof(...)", so I guess there is one positive solution and one negative.

I would like Mapple to give me only the solution that give Citcyt, Citmt, Malmt, and Pyrmt >0.

I tried with the "assuming" argument but I am not sure it is correct:

([solve(eqns, {Citcyt, Citmt, Malmt, Pyrmt}, useassumptions)], assuming [Citcyt > 0, Citmt > 0, Malmt > 0, Pyrmt > 0])

Does somebody knows how to do this ?