Using the phosphatase inhibitor calyculin A, we have examined the influence of phosphorylation on synaptic transmission and plasticity in rat CA1 hippocampal slices. Bath application of 0.5-1 microM of calyculin A resulted in an increase of 42.6 +/- 2.9% in synaptic responses. The effect produced by calyculin A was not accompanied by changes in fibre volley, was not associated with changes in paired-pulse facilitation, and could be reproduced by intracellular injection of the compound, thereby indicating a postsynaptic action. Also, the synaptic enhancement produced by calyculin A was expressed only by potentials mediated by amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, but not by the NMDA responses recorded in the presence of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and low magnesium. The effect of calyculin A could be prevented by KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase II. Long-term potentiation could still be induced in the presence of calyculin A, but the effect of the compound was slightly reduced on potentiated compared with control pathways. These results indicate that calyculin A can selectively increase the efficacy of AMPA receptor-mediated synaptic transmission at excitatory synapses.