The objective of this study was to determine changes in plasma zinc over a 24-hour period and the relationship of these changes to meals. Hourly plasma zinc concentrations were measured in 12 normal adult women for 24 consecutive hours. The subjects were fed standard meals at 0700 hours, 1200 hours and 1700 hours. A decline in plasma zinc started 1/2 to 1 1/2 hours after each meal, reaching a low point 3-4 hours after each meal. During the night there was a progressive increase in plasma zinc concentrations averaging (1.54 micrograms/dl)h-1 for 11 hours. The lowest value of the day at 2100 hours was 22% lower than the highest value at 0730 hours. These results indicate that meals are the primary factor and could be the only factor responsible for variations in an individual's plasma zinc during any one day.