The proportions of fibres of the collagenous and elastic systems were measured in the aortas of three normal heifers and in nine heifers given the calcinogenic plant Solanum glaucophyllum for 15, 30 or 60 days. There were decreases in the amount of collagen relative to reticulin, and in the proportion of elastic fibres which were related to the period of dosing. These changes may have an adverse influence on the animals' cardiovascular function.