Changes in dry weight, osmotic potential, abscisic acid and free proline contents were measured during cold hardening of nine winter wheat varieties differing in freezing resistance. During the first weeks of cold hardening dry weight and proline levels increased and the osmotic potential decreased parallel to the development of freezing resistance. Dry weight reached a broad maximum between the seventh and tenth week of hardening. ABA levels had a sharp maximum around the seventh week of hardening, when the dry weight increase began to cease. Maximal levels of proline were observed seven to ten weeks after the start of cold hardening. The mean and final dry weights, the mean, maximal and final proline contents, as well as the maximal ABA contents of the nine varieties correlated significantly with freezing resistance measured at the end of the hardening period. The possible use of proline and of ABA as biochemical markers for freezing resistance in winter wheat breeding is discussed.