The right ulnar length was determined using a condylograph at 7 set times of each day during a study period of 3 wk. The mean standard error of the determinations amounted to 0.09 mm. In 86% of the days studied the ulnar length changed significantly (1 way analysis of variance P < 0.05). The ulnar length decreased significantly for the whole group during the time interval from 8.00 h to 12.00 h (Friedman''s related samples test, P < 0.01). The decrement amounted to 0.4 mm. During the rest of the day the changes in the ulnar length were not significant. A consistent decrement of the ulnar length during the day in combination with total growth of 0.99 mm during the 3 wk study period suggested that growth in the length of the human ulna occurs during the night.