Small but statistically significant increases in serum total calcium and serum inorganic phosphorus concided with repeated onsets of psychotic agitation or mania in nine psychotic in-patients experiencing rapid cycles of illness. These increases were not accompanied by changes in magnesium or other constituents, which might suggest non-specific haemoconcentration. Similar increases in calcium or phosphorus were not present in patients without the same cycles of psychotic illness. The observed increases could neither be simulated nor altered by stress or activity, and it remains unclear whether they might be accounted for by dietary changes, sleep disruption, circadian phase shifts or by endocrine alterations.