Regional brain tissue prostaglandin (PG) levels have been measured during ischaemia produced by bilateral carotid occlusion for 1 hour and following restoration of flow. In the normal gerbil, the frontal cortical levels of PGF2 alpha were: 6.7 +/- 1.3 pg/mg and for PGE2: 6.4 +/- 1.1 pg/mg of brain tissue protein. Following 1 hour of ischaemia PGF2 alpha rose to 50.4 +/- 8.3 pg/mg whilst there was only a slight rise in PGE2 (10.7 +/- 1.6 pg/mg). Post ischaemic values for parietal and occipital areas were somewhat higher, but showed the same trend. Within 15 minutes of the restoration of flow there was a massive increase in PGF2 alpha levels which reached a peak at 2 hours (300 pg/mg) and then subsided to control values. PGE2 levels did not change for the first 30 minutes of recirculation, but then rose for the rest of the period of observation. The pattern of cytotoxic oedema resembled PGF2 alpha closely while the Evans blue staining (vasogenic oedema) was similar in time to the PGE2 pattern.