Platelet factor 4, which has a potent affinity for heparin, has been shown to inhibit the binding of basic fibroblast growth factor to the cell surface receptor and to counteract the biological activities of basic fibroblast growth factor in certain peripheral tissues. In the present in vitro [125I]basic fibroblast growth factor binding experiments, platelet factor 4 consistently inhibited the binding of iodinated basic fibroblast growth factor to cell membranes of the gerbil hippocampus. To investigate the in vivo function of endogenous basic fibroblast growth factor and/or basic fibroblast growth factor receptor possibly activated in the ischemic gerbil brain, we infused platelet factor 4 continuously into the left lateral ventricle with an osmotic minipump. When platelet factor 4 infusion was started within three days after a 3-min ischemic insult, it significantly enhanced ischemia-induced learning disability and ischemic neuronal loss in the CA1 region of the hippocampus, as demonstrated by the results of the step-down passive avoidance task and by subsequent histological examinations. Infusion of platelet factor 4 into the cerebral ventricle of intact gerbils did not affect learning ability or CA1 neuron number. Basic fibroblast growth factor-neutralizing antibody, when infused continuously in the cerebral ventricle, also exhibited a neurotoxic effect in ischemic but not intact gerbils. Basic fibroblast growth factor co-infused with heparin, but not basic fibroblast growth factor alone, rescued a significant number of ischemic neurons which were destined to degenerate without the infusion of heparinized basic fibroblast growth factor, and it prevented ischemia-induced learning disability.(ABSTRACT TRUNCATED AT 250 WORDS)