Abstract: The phosphorylation state of cp20, a low molecular weight membrane-associated GTP-binding protein, was previously shown to increase two- to threefold 24 h after associative conditioning. Here, cp20 is shown to be phosphorylated by protein kinase C (PKC) in vitro. Pronounced differences in activity were observed with the three major isoforms of PKC, whereas casein kinase, calcium/calmodulin-dependent protein kinase II, and cyclic AMP-dependent protein kinase produced no detectable phosphorylation of cp20. Phosphorylation of cp20 had no effect on its GTPase or GTP-binding activity but caused a translocation of cp20 from cytosol to the nuclei/mitochondrial particulate fraction. These results suggest that the increase in phosphorylation of cp20 after conditioning may be due to PKC.