Caffeine was used as a tool to investigate whether the sarcoplasmic reticulum (SR) properties in single. mechanically skinned fibres dissected from soleus muscle of spontaneously hypertensive rats (SHRs) differ from those in fibres of the same type from age-matched, normotensive Wistar-Kyoto (WKY) rats. The fibres were typed electrophoretically based on myosin heavy chain (MHC) isoform composition. Here we show evidence that the ratio between the caffeine thresholds for contraction at maximal and endogenous resting SR-Ca2+ (Rcaff-th) can be used as an indicator for distinguishing between slow-type SR (Rcaff-th>or =0.73) and fast-type SR (Rcaff-th<0.73). Based on this indicator, 47.5% of the SHR-soleus fibres identified as type I displayed fast-type SR characteristics and 40% of the SHR-soleus fibres identified as type II displayed slow-type SR characteristics. This result explains the shorter contraction and faster relaxation of soleus muscles in SHRs and also suggests that SR with fast-type characteristics can co-exist with slow-twitch MHC isoforms and vice versa.