Hello,   I am quite sure I had found a paper yesterday using CAR within PROC NLMIXED. I am searching for 10 à 15 minutes now (to find it again), but cannot spot it anymore. Unfortunately I do not have an example of this type of model using PROC NLMIXED readily available.    Maybe @jiltao can help you out?   By the way, is this on pharmaco-kinetics and compartmental models? If yes, this is FYI. ... ... Currently, NLMIXED is indeed the best option for nonlinear mixed effect models; however, it’s support for ordinary differential equations used in pharmacokinetic models is limited to 1, 2, and 3 compartment models with specific dosing (aka forcing functions).  I believe SAS is working on adding support for more general pharmacokinetic models.   Thanks, Koen ... View more