The present study documents daily rhythms of plasma atrial natriuretic peptide, serum osmolality and haematocrit in the rat. One‐hundred and twenty‐five Sprague‐Dawley rats were used. They were bred under a cycle of 12 h light/12 h dark starting at 07.00 h. Fifty‐three rats were decapitated between 09.00 and 16.00 h (study I) and 72 rats in groups of six were decapitated at 2‐h intervals for a period of 24 h (study II). In study I, plasma atrial natriuretic peptide was 156±11 pg mg‐1(mean ± SEM). In study II, atrial natriuretic peptide was at a control level from 08.00 to 18.00 h and then began to increase. At 22.00 h, atrial nartriuretic peptide was 420 ± 105 pg ml‐1, which was significantly higher than at 08.00 h (P<0.05). The serum osmolality was over 300 mosmol kg‐1during the day. The highest mean osmolalities (315, 317, 312 mosmol kg‐1were found from 18.00 to 22.00 h. These were significantly different (P<0.05) from other groups during the day. The haematocrit was highest at 14.00 h (49.5 ± 0.7%) and lowest at 24.00 h (43.6 ± 0.8%) (P<0.05).In conclusion, we have shown that there are significant daily rhythms of plasma atrial natriuretic peptide, serum osmolality and haematocrit during a 24‐h period and 12 h light/12