Age-related changes of the hippocampal mossy fibre system were studied in rats of different ages (4, 12, and 24 months) using the Timm's histochemical technique for the detection of tissue stores of zinc and heavy transitional metals. Quantitative image analysis and microdensitometry techniques were applied for the evaluation of the changes themselves. The area occupied by mossy fibres and the intensity of Timm's staining in field CA3 of hippocampus were greater in adult rats than in young and old animals. These findings are consistent with previous behavioural data indicating that young and old rats exhibited a reduced passive avoidance learning with respect to adult animals. The above techniques were also used in a second session of this study designed to investigate the effect of a 6-months treatment with acetyl-L-carnitine (ALC) on the hippocampal mossy fibre system. ALC was found to increase significantly the area occupied by mossy fibres and the intensity of Timm's staining in the hippocampus of old rats. These findings are in agreement with and support from an anatomical point of view, the data indicating that long-term treatment with ALC improves passive avoidance performance.