AbstractGlycolmethacrylate (GMA) is now a widely accepted plastic embedding medium for histopathological uses although technical conditions of polymerization are not fully understood. We found that benzoyl peroxide initiated GMA medium accelerated with dimethylaniline (NN-DMA) polymerizes at different rates according to the volume left to polymerize and the size of the embedding molds. This phenomenom appears to be sustained by an unexpected and complex thermodynamical process that we were unable to resolve. However, we did find that NN-DMA concentration is a polynomial function of the initial GMA volume, and the proposed equation may help in adjusting the techniques when varying the amount of methacrylate embedding medium or when changing the size of the embedding molds. (The J Histotechnol12:89, 1989.)