Potted citrus trees, 1 yr old ''Valencia'' orange (Citrus sinensis [L.] Osbeck) on 1.5 yr old rough lemon (C. limon [L.] Burm. f.) rootstocks, were maintained for consecutive 7 day periods in controlled environments at progressively colder temperatures from 30 to O.degree. C. Temperatures were decreased abruptly by 5.degree. C intervals, and leaves and stems from selected sample trees were removed for analyses immediately before each temperature decrease. Carbohydrates accumulated most rapidly between 15-5.degree. C. At 10.degree. C, carbohydrates accumulated in the leaves at an average rate of 8 mg .cntdot. g dry wt-1. Slower rates of carbohydrate accumulation of 3 mg .cntdot. g dry wt-1 .cntdot. day-1 were found in the stems. Reducing sugars increased with colder temperatures, but there was little starch hydrolysis above O.degree. C. At O.degree. C, total nonstructural carbohydrates stopped accumulating. O2 uptake, H2O content and chlorophyll decreased with colder temperatures. Freeze tests of -6.7.degree. C for 4 h at the end of the O.degree. C regime indicated some cold hardening of the trees. Results suggested that relatively low starch-sugar conversion restricts maximum cold hardening in citrus at low but nonfreezing temperatures.