I have an experiment with an interaction (VT) between Varieties and Time. There are 12 Varieties and 2 Times (Early and Late), and Time is a fixed affect. I have been estimating a single variance from this interaction using pdIdent(~VT-1). I would like to test 2 more complicated models. 1) Estimate 2 variances, one for Variety*Time(Early) and one for Variety*Time(Late), again with a diagonal matrix. 2) The above with a covariance term between the Times across Varieties. To do this, I think I would want a variance covariance matrix like (1) but with one more parameter wherever a row and a column share a Variety but differ in Times. The rest would be zero. I would appreciate if someone would explain how I can do this using the standard pdMat classes, or, more likely, how to create a new one to do this. Thanks much, Scott Rifkin