Thirty-eight mature horses were assigned to one of two equal groups to evaluate two treatments consisting of either 24 hours of continuous road transport (24T) or two 12-hour periods of transport separated by off-loading, resting and feeding the horses for 12 hours (12/12T). A subset of six horses from each group served as controls for the other group. The horses were loaded into a commercial straight-deck trailer and travelled loose in one of two standard-sized compartments. After the journeys the horses were put back into their paddocks for a 24-hour recovery period. Venous blood samples were collected before loading, after unloading and after the 24-hour recovery period. Transport significantly increased the horses' cortisol concentrations, neutrophil counts and neutrophil:lymphocyte (nl) ratios, and decreased the numbers of all the lymphocyte subpopulation cell types. Collectively, no significant differences were observed between the two treatments in the horses' cortisol concentrations, total leucocyte counts, neutrophil and lymphocyte counts, nl ratios, and the cd8a+ and cd21+ lymphocyte subpopulations, but there were differences in the numbers of cd3+, cd4+, and cd8b+ subpopulations. The inclusion of a 12-hour rest-stop interrupted the transport-related decline in the lymphocyte subpopulations and allowed them to recover towards their resting levels.