The efficacy of the 'EGTA-treatment' for producing a model of selectively 'skinned' cardiac muscle has been questioned. This paper deals with ultrastructural evidence designed to test whether small ions can gain access to the myofibrillar space of the heart after 'EGTA-treatment'. Lanthanum has been employed because of its widespread use as an 'extracellular' marker and because its presence can be unequivocally demonstrated by X-ray microanalysis. The results of both standard transmission electron microscopy and X-ray microanalysis reveal that, despite some deterioration of the ultrastructure after 'EGTA-treatment' at 2 degrees C for 24 h, lanthanum is still apparently excluded from the intracellular spaces. Parallel runs with detergent treatments, such as Triton X-100 and the alkaloid saponin, demonstrate that La3+ is deposited on the contractile proteins in readily detectable amounts in these circumstances. Even in areas of the sections devoid of visible electron-opaque deposits, recognizable by eye in standard transmission images, X-ray microanalysis frequently revealed the presence of La3+. It is concluded that the sarcolemma persists as a selective permeability barrier to small ions such as La3+ and LaEGTA after 'EGTA-treatment'. These findings are complementary to the mechanical behaviour of chemically treated cardiac muscle.