The effects of exhaust gases and distillates of sump oil from old motor cars on the activity of Glossina palpalis (R.-D.) were briefly investigated, using a method described in the preceding paper. The exhaust gases had no effect on the tsetse flies and the threshold and standard concentrations of vapour of distillates from sump oil required to stimulate the tsetse flies decreased as the boiling point of the fractions increased. This result is consistent with the results of experiments by previous workers using series of homologous pure compounds, and suggests that any very stimulating compound could only be present in very small quantities in sump oil.