To explore the relationship between ocular (fundus) hemoglobin and that sampled and measured conventionally. To look for differences in hemoglobin density determined by both methods when the body hemoglobin is acutely (blood donation) or pathologically e.g. anemia altered.Conventional (capillary and antecubital) and ocular fundus (papillary and choroidal) determinations of hemoglobin density in 14 females and 23 males, aged 25 to 30 years were compared. Application of the ocular method before and after blood donation in 21 females and 12 males, aged 20 to 68 years was performed. All these subjects were ophthalmologically and systemically healthy. Five male and 5 female anemia patients, aged 27 to 90 years, were also measured as above.Good correlation between fundus hemoglobin density and capillary (r = 0.81) and venous (r = 0.61) hemoglobin was observed in healthy persons. Differences in hemoglobin density according to gender were obvious at all fundus sites measured. Following blood donation, papillary hemoglobin density in males moreover increased, while that in females decreased (F = 7.53), suggesting a gender-specific difference in the ocular blood regulation, an effect also noted in the anemia patients.Comparison of conventional and ocular determination of hemoglobin reveals good correlation in healthy people. However, in acute or chronic blood loss the papillary hemoglobin level differs from that measured peripherally. A gender-related regulatory capacity of the ocular tissues under low-level conditions can be shown: Male persons maintain ocular hemoglobin at a normal level even when peripheral hemoglobin falls to low values, whereas female persons show a decrease in ocular hemoglobin parallel to the venous levels. Hence, under such extreme conditions,--and only in males--the ocular method yields values other than those from the conventional method, because ocular regulatory mechanisms, otherwise undetected, are exquisitely revealed.