Two independent, excitatory, monosynaptic afferent fiber systems projecting onto basal and apical dendritic layers of the CA1 sub-field of the hippocampus were tested for interactive effects. Long-term potentiation of either of the pathways was accompanied by a transient depression in the other pathway. Similar transient depression could be elicited by antidromic stimulation of the CA1 cells. No long-term heterosynaptic effects were observed.