Pea (Pisum sativum L. cv. Feltham First) plants were germinated and grown under two temperature regimes, one chilling (6-8ยฐ C) and one non-chilling (16-18ยฐ C), which are referred to as "cold-grown" and "warm-grown", respectively. It was found that: (1) At saturating light intensity and with excess CO2, cold-grown leaves exhibited faster rates of oxygen evolution than warm-grown leaves when measured below 15ยฐ C. However when measurements were carried out above this temperature, the reverse relationship was observed. (2) Full-chain electron-transport measurements on thylakoids showed that those isolated from cold-grown plants had greater light-saturated uncoupled rates than their warm-grown equivalents at all temperatures between 3 and 19ยฐ C. (3) This difference was apparently not due to a greater activity of photosystem I or II in the thylakoids from cold-grown plants, but rather to a more rapid turnover of a dark step within the electron-transport chain. These results are interpreted in terms of a previously reported apparent homeoviscous adaptation of the pea thylakoid membrane to growth temperature (J. Barber, R.C. Ford, R.A.C. Mitchell, P.A. Millner, 1984, Planta 161, 375-380).