Four soluble and two cell wall bound peroxidases account for most of the peroxidase activity in sections of tomato stem tissues. These peroxidases differ in heat sensitivity when incubated in acidic media. Two of the six peroxidases are inactivated at higher temperatures in tissue sections than in tissue extracts treated in the same way. Under defined conditions of heating, all but one of the tomato peroxidases can be inactivated in tissue sections and extracts. A cytochemical peroxidase method is described and used to determine the cellular distribution of this heat-stable peroxidase in developing stem tissues. Changes in the activity and cellular distribution of this peroxidase occur in tissues of the stem during development.