This study was designed to establish the in vitro pharmacological responses of rat middle cerebral arteries (MCA), and also their susceptibility to overnight cold storage at 4°C. MCAs were harvested from rats and pharmacological responses were studied using a Mulvany myograph. Responses to prostaglandin F2<sub>α</sub> (PGF<sub>2α</sub>), uridine triphosphate (UTP), noradrenaline (NA) and histamine were determined to investigate receptor-dependent responses, sodium nitroprusside and papaverine to investigate receptor-independent relaxation and L-arginine and Nω-Nitro- L-arginine methyl ester ( L-NAME) to examine nitric oxide synthase-dependent responses. Responses in fresh arteries were established and compared with responses in arteries which had been mounted on a myograph and stored overnight at 4°C before study the next day. The MCAs had an effective lumen diameter of 263 µm (SD 25) and sustained concentration-dependent contractions were produced by 124 m M K<sup>+</sup>, PGF<sub>2α</sub>, UTP and L-NAME. Sodium nitroprusside, NA, histamine, papaverine and L-arginine produced concentration-dependent relaxations in precontracted arteries. Overnight cold storage did not alter the pharmacological responses to any of the agonists tested.