Norepinephrine-containing storage particles in rat heart have been disrupted to determine their stability and to quantitate the amount of dopamine beta-hydroxylase (DBH) that exists in a soluble, membrane enclosed pool. Vesicles were relatively resistant to lysis by hypotonic shock or by up to five cycles of freezing and thawing. These procedures have been shown to release catecholamines from adrenal chromaffin granules readily. However, rat heart vesicles were sensitive to temperature and they could be lysed by mechanical disruption using a sonicator or a Polytron homogenizer. Increasing the force or duration of the disruption resulted in an increased release of norepinephrine and DBH into a supernatant fraction after high-speed centrifugation. Extrapolation of the data from such experiments gave an estimate for membrane-enclosed DBH of 37% of the total amount of enzyme activity. This estimate of the proportion of membrane-enclosed enzyme is considerably higher than those previously reported for membrane-enclosed DBH in sympathetic neurons.