In order to elucidate the role of NO in LTP, we have investigated a biochemical effector of NO action at hippocampal synapses, guanylyl cyclase. We have observed that LTP-inducing stimuli elicit an increase in the activity of guanylyl cyclase, an effect blocked by inhibitors of NO synthase. Extracellular application of hemoglobin, which binds NO and thereby blocks its actions, also attenuated the increase in guanylyl cyclase activity. Taken together, these results provide direct biochemical evidence for an elevation of NO levels with LTP-inducing stimulation, and support the hypothesis that NO can function as a transcellular messenger in the hippocampus. These findings also implicate guanylyl cyclase as a target of NO and demonstrate that while NO is elevated with LTP-inducing stimuli, the activity of NO synthase is not necessary for induction of LTP by all stimulus paradigms.