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Department of Physiology, Monash University, Clayton, Victoria, Australia.

1. Isometric contractile characteristics of fast-twitch (flexor digitorum longus, FDL; medial gastrocnemius, MG) and slow-twitch (soleus) muscles were determined in pentobarbitone-anaesthetized fetal sheep between 90 and 140 days gestation. Five fetuses were hypophysectomized (HPX) at 90-95 days gestation and then studied at 138-140 days. 2. At 90-95 days gestation the time to peak of single twitch contractions for the soleus, MG and FDL were not significantly different from each other; the mean value (+/-S.E.M.) for all the muscles at this age was 77.6 +/- 9.0 ms. At 120-125 days gestation the MG and FDL contracted significantly faster (44.0 +/- 0.9 and 40.8 +/- 1.8 ms, respectively) than at 90-95 days, and did not change significantly thereafter. In contrast, the soleus muscle contracted more slowly (111.9 +/- 6.6 ms) at 138-140 days than at 90-95 days and 120-125 days gestation. 3. Soleus muscle consisted of type I fibres at all gestational ages. There was no significant change with gestational age in the relative numbers of type I and II fibres in the MG and FDL, but in the diaphragm the number of type I fibres increased and the number of type II fibres decreased between 125 and 138 days gestation. 4. HPX abolished the normal increase of soleus weight relative to body weight between 125 and 138 days but did not alter the change of twitch contraction time with age. HPX significantly prolonged twitch time to peak and time to half-relaxation of MG and time to half-relaxation of FDL at 138 days. 5. The maximum rate of rise of the isometric tetanic contraction was unchanged by HPX in all three hindlimb muscles, but fatigue of MG and FDL was increased. 6. The relative proportions of different fibre types in the hindlimb muscles and the diaphragm were unchanged by HPX, but there was a significant decrease in mean areas of type I and II fibres in the FDL and MG of the HPX fetuses.