Rb-Sr isotope data of whole-rock samples and apatites from the Hällefors composite dyke, south central Sweden, are presented and discussed. A good fit of several whole-rock points on a 1000 Ma line suggests a homogeneous Rb-Sr composition along the Hällefors dyke. The obtained data (age = 1001 ± 8 Ma, initial Sr ratio = 0.7103 ± 0.0003 and MSWD = 2.84) most likely reflect the low-grade metamorphism or alteration to which the rocks of the dyke were subjected. Several factors, besides sample inhomogencity, are suggested to explain the scattering of the remaining whole-rock samples around the 1000 Ma line: (a) variations inherited from the primary distribution of Sr and Rb in the different components of the dyke, (b) variations due to sampling within more altered domains of the dyke, (c) inheritance of the Sr isotope ratio from the country-rock gneiss. Two apatite separates of different rock types give a mean87Sr/86Sr ratio of 0.70505 ± 0.00043, believed to reflect the initial Sr ratio of the Hällefors system.