Brief localized heating of cotton cotyledonary petioles increased phloem callose and decreased lateral movement of 14C-assimilates. There was little or no increase in phloem callose when plants were heated directly after a dark period of 16 or more hours. Such heated low-callose petioles had more lateral movement than heated high-callose petioles. Phloem callose and lateral movement returned to normal within 1 day after heating. These results suggest that phloem callose caused part of the decrease of lateral movement in heated high-callose petioles. Excessive phloem callose would be expected to restrict lateral movement, if movement from sieve tubes were via plasmodesmata.