1. The contractile properties of soleus and extensor digitorum longus muscles from animals at 12 months of age whose mothers had been undernourished during pregnancy and lactation (PU rats) have been compared with those of muscles from ge-matched controls. 2. Body-weight and muscle wet weight of PU rats was significantly reduced. Muscle:body-weight values were, however, no different from controls. 3. No significant alterations in whole muscle speeds of contraction or relaxation could be detected when compared with those of age-matched controls. 4. Twitch and tetanic forces of both SOL and EDL were greater per unit weight of muscle in PU rats. Apart from SOL twitch these differences were significant. The tetanus:twitch values were, however, not different.