This study examines the pathophysiology of stroke secondary to focal cerebral ischemia. The interaction of arachidonic acid metabolites and polyamines, a class of ubiquitous ornithine-derived molecules with important membrane effects on edema, Caplusplus-dependent endocytosis, platelet function, and prostaglandin (PG) formation, are correlated with regional changes in H2 clearance, cerebral blood flow (rCBF), ischemic edema, and somatosensory evoked responses (SSERs) after middle cerebral artery (MCA) occlusion. Thirty cats were studied up to 3 hours before and 6 hours after right MCA occulsion. Four areas of brain showing different levels of perfusion after MCA occlusion were sampled for tissue levels of PGs: 6-keto-PGF1a, PGE2, and as well as thromboxane B2 (TXB2), ornithine decarboxylase activity (ODC) (a measure of polyamine activity) and gravimetric determination of cerebral edema. After right MCA occlusion, right hemisphere SSER amplitude decreased and interpeak latency increased markedly. rCBF was distributed into zones of dense, partial, and no ischemia ranging from 12.6 to 59.4 ml/100g/minute. Ischemic edema was distributed inversely to rCBF and was increased in areas of dense ischemia (85.2 + 0.5%) and ischemia (82.7 + 0.7%), but not in partially ischemic or control areas. 6-Keto-PGF1a (1257.3 pg/mg), PGE2 (1628.5 pg/mg), and TXB2 (1572.8 pg/mg) were all significantly (P < 0.05) increased in areas of partial ischemia that had not yet developed edema. ODC levels were significantly elevated (3812 pmole/g/hour, P < 0.05) and increased with time in areas of slightly denser ischemia that showed an intermediate increase in edema, but not the presence of infarction. This is the first demonstration that ODC, the rate-limiting enzyme for polyamine synthesis, is stimulated by cerebral ischemia. This stimulation occurs in areas of incomplete ischemia that are developing edema and occurs without a marked increase in tissue PGs. This observation suggests that polyamines may play a pivotal role in determining functional integrity in zones of partial ischemia surrounding a developing cerebral infarct.