Dear Group Members, I am trying to model one PK/PD data in phoenix. PD data is showing a time lag of two hour. I want to capture the time lag by using “IF and ELSE” kind of statement, which is k/a “Sequence/sleep” statement in Phoenix. How to defined flag/add declaration before equence/sleep statement? I tried but always giving responce like variable undefined??? Here is the code: } test(){ deriv(A1=-kaA1) deriv(A2=kaA1-kelA2) deriv(A3=kng-((EmaxC)/(EC50+C))* A3) sequence{ flag=0; sleep(2); flag=1;} C=A2/V dose point (A1) error(CEps=1) observe(CObs = C+CEps) stparm(V = (tvV)) stparm(kel = (tvkel)) stparm(ka= (tvka)) stparm(ka= (tvka)) fixef(tvV = c(, 3.8, )) fixef(tvkel = c(, 1.24, )) fixef(tvka = c(, 0.5, )) fixef(tvkng = c(, 0.68, )) } I would appreciate guidance and appropriate suggestions to this problem. Regards Raj.