A quantitative histological study of neurons in the locus coeruleus (LC) was carried out in ASH/TO mice aged 6, 9, 12, 15, 22, 25, 28 and 31 months. Counts were carried out on the right LC in 6 /xm parasagittal sections. There was a wide individual variation and also a variation between age groups from 6 and 15 months, but the mean number of neurons in the LC never fell below 1325 until after 25 months. The mean number of neurons in the LC from 6 to 25 months was 1520 ± 73. The mean number of neurons at 28 and 31 months was 1009 ± 60 and 854 ± 146 respectively. This indicates that the pattern of neuron loss in the LC of ASH/TO mice is similar to that found in the normal ageing human brain in a number of quantitative histological studies.