Kinetic studies of the inhibition of rabbit muscle phosphorylase a D-glucan:orthophosphate alpha-D-glucosyltransferase (EC 2.4.1.1) by various glucose derivatives indicate that glucose derivatives in the chair conformation (glucose, 2-deoxyglucose, glucosyl fluoride, and 1-O-methylglucose) stabilize the "T" form of phosphorylase a, whereas those in the half-chair conformation (1,5-gluconolactone and glucal) bind to the "R" form. Derivatives which stabilized the T form were generally competitive inhibitors of phosphate binding, were synergistic with caffeine, and weakened AMP binding. Derivatives which bound to the R form were noncompetitive with respect to Pi in the presence of AMP. The mechanism of inhibition of the R form by gluconolactone and glucal appears to depend upon the presence or absence of AMP. In the presence of the nucleotide, gluconolactone is probably a linear mixed-type inhibitor. However, in the absence of AMP, simple linear uncompetitive inhibition was apparent. Since gluconolactone is thought to be an analogue of the substrate transition state of the phosphorylase reaction, a major function of nucleotide in activating phosphorylase may be the formation of the transition state binding pocket in the binary enzyme-glycogen complex. In the absence of AMP, glucal was competitive with respect to phosphate. However, in the presence of nucleotide, simple intersecting linear noncompetitive inhibition was observed. The results indicate that a second function of AMP is the formation of the phosphate binding pocket in the enzyme-glucal-glycogen-AMP complex. This conclusion is consistent with the crystallographic and structural data available at present.