To investigate the mechanism by which developmental iodine deficiency and hypothyroidism affect long-term synaptic plasticity, we chose to study the activity of protein kinase C (PKC) and the expression of growth-associated protein-43 (GAP-43) in pup hippocampus following developmental hypothyroidism. Two developmental hypothyroid rat models were created with iodine-deficient diet (IDD) and water containing methimazole. Compared to the control group, in developmental hypothyroid rats: (i) HFS induced less in vivo LTP and more long-term depression (LTD) in area CA1 on postnatal day (PN) 60 (P < 0.0 5); (ii) in IDD pups, HFS-induced LTP showed much smaller population spike amplitude (P < 0.01) and slope of field-excitatory postsynaptic potential (P < 0.01); (iii) PKC activity, on PN30, was much higher on cell membrane and lower in cytosol (P < 0.05) without change of total PKC level (P > 0.05); and (iv) GAP-43 expression was significantly lower (P < 0.01) on both PN30 and PN60. Developmental iodine deficiency and hypothyroidism affect PKC translocation, decrease GAP-43 expression, and impair long-term synaptic plasticity in the rat hippocampus.