1. In pineal glands of adult spontaneously hypertensive rats (Okamoto strain), dopamine levels increased 34%, adrenaline concentrations increased 152% and noradrenaline levels decreased by 26%, when compared with Wistar-Kyoto normotensive controls. 2. These results are consistent with the hypothesis of an increased uptake of peripheral adrenaline resulting in increased release of noradrenaline from pineal sympathetic nerves in hypertensive rats. 3. The activity of pineal N -acetyltransferase was increased fourfold in hypertensive rats, indicating increased stimulation of pineal β-adrenoreceptors. 4. Methylation of pineal membrane phospholipids was also increased (100-320%) in hypertensive rats. 5. These results indicate a correlation in vivo between increased β-adrenoreceptor stimulation and increased methylation of membrane phospholipids in the rat pineal gland, which could result in changes in membrane fluidity and affect the coupling of the β-adrenoreceptors to the adenylate cyclase.