1. Field potential recordings were used in rat motor cortex (MI) slice preparations to investigate the potential for activity-dependent modifications in the effectiveness of synaptic connections formed by layer II/III horizontal projections. 2. Long-term potentiation (LTP) of synaptic efficacy in MI horizontal pathways could be produced at short (0.5 mm) and long (1.0 mm) distances by theta burst stimulation (TBS), but only during local, transient application of the gamma-aminobutyric acid-A (GABAA) receptor antagonist, bicuculline methiodide (bic) immediately before TBS. Mean increase of the field potential amplitude measured 25-35 min after LTP induction ranged between 24 and 34%. Cooperative effects of conjoint TBS of two horizontal pathways on LTP induction were observed. 3. These results demonstrate that persistent changes in the functional interactions of cortical neurons can arise by activity-dependent mechanisms within the local horizontal connections and suggest a likely mechanism to recognize cortical representation patterns.