The endocrine response of porcine parathyroid gland tissue slices in vitro to aluminum was studied by electron microscopy and radioimmunoassay of PTH. Medium aluminum concentrations were 20 to 500 ng/ml covering the range corresponding to concentrations reported in the plasma of aluminum-intoxicated hemodialyzed patients. Aluminum inhibited iPTH-release and caused severe cell alterations. This inhibition was incomplete and there was an aluminum-insensitive iPTH-release capacity. This phenomenon seemed to be due to heterogeneous parathyroid cell population as regards aluminum sensitivity, perhaps linked to the spontaneous asynchronous cyclic parathyroid cell changes. Sensitivity to aluminum was modulated by the extra-cellular calcium concentration. Sensitivity to extra-cellular calcium concentration variations persisted in aluminum intoxicated tissues. The severity of the observed cell lesions induced by high concentrations of aluminum suggested that the recovery of an iPTH-release capacity when parathyroid tissue was withdrawn from a toxic environment and switched to aluminum-free media is more likely to be due to activation of a "less-sensitive to aluminum" cell pool than to a true reversibility of the toxic effect.