proc model data=compart;
   endogenous conc;                                   
   exogenous time;                                    
   parms a1 95.0 a2 120.0 theta1 1.5 theta2 0.12;     
   conc=(a1*exp(-theta1*time))+(a2*exp(-theta2*time));
   resid.conc=resid.conc/(sqrt(pred.conc));           
   fit conc;                                          
run;                                                  
quit;