Responses of methanol-extractable UV-B-absorbing compounds in needles of mature Scots pines (Pinus sylvestris L.) to enhanced UV-B were studied in a field experiment at the University of Oulu. The needle samples were collected in September, 1996 (3-month-old needles), and three times in 1997: in June (11-month-old needles), in July (one-month-old needles) and August (2-month-old and 14-month-old needles). The concentrations of UV-B-absorbing compounds differed significantly between needles of different ages, but no differences were observed between ambient and supplemental UV-B treatments. However, older needles contained slightly more compounds in supplemental UV-B compared to ambient treatment, whereas younger needles contained smaller concentrations of these compounds in supplemental UV-B.