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;