ALTHOUGH lunar crystalline rocks are essentially devoid of nitrogen, the same is not true of the lunar regolith. The nitrogen contents of individual regolith samples (which can be as high as 0.012% by mass) correlate strongly with abundances of noble gases known to be implanted in the lunar surface by solar radiation, indicating that lunar regolith nitrogen is also predominantly of solar origin(1). The large variability in N-15/N-14 ratios measured in different regolith samples may thus reflect long-term changes in tbe isotopic composition of the solar radiation(1). But attempts to explain these variations have been hampered by the lack of any firm constraint on N-15/N-14 in, the present solar wind. Here we report measurements of nitrogen isotopes from two lunar samples that have had simple (and relatively recent) exposure histories. We find that nitrogen implanted in the lunar surface during the past 10(5) to 5 x 10(7) years has a N-15/N-14 ratio approximately 40 parts per thousand higher than that in the terrestrial atmosphere, which is substantially lower than most previous estimates(2,3). This isotopic signature probably represents the best measure of N-15/N-14 in, the present-day solar wind.