The concentration of particulate lead in the atmosphere in the central reserve of the M4 motorway at Harlington was measured continously over the period 1 March 1973 to 27 May 1975 in order to determine the long term trends and any seasonal fluctuations in the levels. The lead content of the air was shown to have decreased by 7 per cent per annum and a seasonal cycle existed which could be attributed almost entirely to variations in wind speed. The legislation reducing speed limits in the winter of 1973-74 led to a marked reduction in lead concentration in the short term. The concentration of lead was measured inside two houses, one with window ventilation facing the motorway side. The lead concentration in the first was the same as that expected outside the house, and the second had a concentration of one third of that expected outside. The lead concentration behind a motorway noise barrier was measured to assess the effect of the barrier on lead levels. It was found that the lead levels with and without the barrier in place were not statistically significantly different.