IN 3 EXPERIMENTS SERIES OF SUCCESSIVE ESTIMATES (PRODUCTIONS) OF 1-MIN PERIODS WERE REQUIRED OF HUMAN SS UNDER RESTING (R) AND EFFORTFUL (E) CONDITIONS. IN EXP. I AND II AL L SS SERVED IN BOTH CONDITIONS, THEIR SUCCESSIVE PRODUCTIONS BEING MADE ALTERNATELY UNDER R AND E. IN EXP. III INDEPENDENT GROUPS SERVED IN R AND E, BUT ALL PRODUCTIONS WERE MADE DURING A RESTING STATE. MEAN TIME-PRODUCTION ERROR MARKEDLY INCREASED AS A FUNCTION OF SUCCESSIVE JUDGMENTS IN ALL 3 EXPERIMENTS, UNIFORMLY IN THE DIRECTION OF INCREASINGLY LONGER JUDGMENTS. IN EXP. II AND III JUDGMENTS WERE SIGNIFICANTLY LONGER IN E THAN R, AND THIS EFFECT WAS GREATER IN EXP. III THAN II. IN EXP. I AND II, INTERRUPTION OF THE SEQUENTIAL TASK WAS FOLLOWED BY A TEMPORARY REDUCTION IN THE MAGNITUDE OF JUDGMENT ERRORS. DISCUSSION INCLUDED THE POSSIBLE SIGNIFICANCE OF THESE RESULTS WITH RESPECT TO PROPERTIES OF A PRESUMED TIME-KEEPING MECHANISM. (PsycINFO Database Record (c) 2006 APA, all rights reserved)