Fatigue and circadian rhythms have been proposed as explanations of ti
me-of-day effects on retrospective memory. Both explanations predict p
oor morning performance, a midday performance peak, and then declining
performance and poorest recall in the evening. Whereas this pattern h
as been found in research on retrospective memory using ordered serial
recall, in the single relevant study on prospective memory, performan
ce was found to be highest in early morning, followed by a midday decl
ine with no additional declines. Using post hoc analyses, we investiga
ted older adults' prospective memory throughout the day. Data were tak
en from two studies we had conducted previously and one recently compl
eted experiment. In each of the three experiments, we examined simulat
ed medication and appointment adherence over a 13-day period and found
prospective memory to be better in the morning than at midday. In two
experiments, we found no further decline after midday, and iii the th
ird experiment, performance actually increased in the evening compared
with midday. These post hoc analyses provide preliminary evidence tha
t factors different from or at least in addition to fatigue and circad
ian rhythms produce time-of-day effects on prospective memory, indicat
ing the need for more programmatic research in the future. One explana
tion for these findings is that attentional capacity devoted to prospe
ctive-memory task varies inversely as a function of activity level dur
ing different times of the day.