The effect of competition and the influence of age and sex on performance were examined in a study of 18 Thoroughbred racehorses. The horses performed two solo and two competitive runs at 1,200 and 1,600 m for a total of eight runs. No group ran faster during competition, which may have been a reflection of the quality of horses used for this study and their susceptibility to stress-induced impairment of performance. Males showed no significant difference between competitive and solo run times, whereas females were consistently slower during competition. Males ran significantly faster than females in all runs. There was no difference in run times due to age, which may have been due to the high mean age (5.9 yr) of the group. The slower competitive run times may have occurred because of an earlier onset of fatigue when compared with solo runs. Plasma lactate was significantly greater for the 1,200-m competitive than for the solo runs.

t All latitudes herein mentioned are south, and all elevations are in English feet and dated from sea-level, unless otherwise stated. I shall use the word Cordillera to represent the single chain or cord, the long main ridge or Cordillera of the Andes, the continuous backbone of the continent, and shall not apply the term Cordillera to the inland range of the Andes or to any of its numerous ramifications. All "miles" are English statute miles.

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