We reported that when synthase D was converted to synthase I in a rat liver extract, it progressed through a synthase form with activity characteristics which could not be explained by a mixture of synthase D and synthase I (Tan, A. W. H. (1981) Biochem. J. 200, 169-172). In this study we will borrow the "R" nomenclature to describe this "non-D" and "non-I" activity. Using activities measured at five different conditions and simultaneous equations, the amount of the three synthase forms in liver extracts can be estimated. During incubation of the liver extract, the amount of synthase R was found to increase with time and then to decrease as synthase I was generated, a profile typical of an enzyme intermediate. We investigated for the presence of synthase R in rat liver under different in vivo conditions. In contrast to the liver of fed rats which had very little synthase R, the liver of fasted rats was found to have 30% of its synthase in the R form. This synthase R was increased 2-fold when glucose was given and decreased to a very low level when glucagon was given. Synthase I was not detected, even in the livers of starved rats given glucose. Using conditions which were closer to those of the cell, synthase R was found to have relatively high activity, up to 70% that of synthase I. Based on these results, synthase R is proposed to be an active enzyme form responsible for glycogen synthesis in rat liver.