The frost hardiness of many plant species can be increased by exposing plants to low, non-freezing temperatures. It has been shown that at least in some herbaceous mono-and dicotyledonous species, hardening can also be induced by treating plants with NaC1 at otherwise non-hardening temperatures. In the present investigation, the roots of approximately six-week-old spinach (Spinacia oleracea L.) seedlings were exposed to a 300 mM NaC1 solution. Frost hardiness of the leaves, measured by the electrolyte-leakage method, increased by 2.3 ~ over a 24-h salt-stress period. Salt uptake, as measured with a chloride-sensitive electrode in leaf homogenates, was rapid over the first 7 h of salt exposure and then slowed down. There was no correlation between the chloride content of the leaves and their frost hardiness. While the electrolyte-leakage measurements gave an estimate of damage inflicted on the plasma membrane, plastocyanin release was measured to probe the intactness of the thylakoid membrane system after a freezethaw cycle. The frost hardiness of the thylakoids in situ increased by 4 ~ over 24 h of salt stress to the seedlings. Over the first 5 h, hardening was six times faster in thylakoids than in the plasma membrane. This rapid increase in hardiness in vivo was reflected in reduced in-vitro freeze-thaw damage to thylakoids isolated from seedlings after only 1 h of salt stress.