data compart;                                         
   input time conc @@;                                
   cards;                                             
0.25  177.3                                           
0.50  175.1                                           
0.75  140.8                                           
1.00  145.2                                           
1.50  110.0                                           
2.00  110.0                                           
3.00   95.9                                           
4.00   84.9                                           
6.00   67.3                                           
12.0   30.8                                           
24.0   13.2                                           
48.0    2.7                                           
;                                                     
                                                      
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));
   fit conc;                                          
run;                                                  
quit;