Daily creatine supplements (0.258 g kg(-1) ) were administered to adult male Wistar rats (n = 7) in the drinking water. Age matched rats (n = 6) acted as controls. After 5-6 days, contractile properties were examined in soleus and extensor digitorum longus (EDL) muscle strips in vitro at 30 degrees C. In soleus muscles, creatine supplements decreased the half-relaxation time of the isometric twitch from 53.6 +/- 4.3 ms in control muscles to 48.4 +/- 5.5 ms but had no effect on twitch or tetanic tension or on twitch contraction time. In EDL muscles twitch tension, tetanic tension, twitch contraction and half-relaxation times were all unaffected by creatine supplements. Creatine supplements increased the fatigue resistance of the soleus muscles but had no effect on that of the EDL muscles. After a 5 min low-frequency fatigue test, tension (expressed as a percentage of initial tension) was 56 +/- 3 % in control soleus muscles, whereas that in the creatine-supplemented muscles was 78 +/- 6 % (P < 0.01). In the EDL muscles, the corresponding values were 40 +/- 2 % and 41 +/- 9 %, respectively. The force potentiation which occurred in the EDL muscles during the initial 20-30 s of the fatigue test was 170 +/- 10 % of initial tension in the control muscles 24 s after the initial stimulus train but was reduced (P < 0.01) to 130 +/- 20 % in the creatine-supplemented muscles. In conclusion, soleus muscle endurance was increased by creatine supplements. EDL endurance was unaffected but force potentiation during repetitive stimulation was decreased. Experimental Physiology (2001) 86.2, 185-190.