Changes in osmolality during liquefaction of human semen.

The osmolality of 18 liquefying human semen samples from 15 volunteers was measured by vapour pressure osmometry to be low (294 mmol/kg, range 269-311). For each sample the osmolality increased during liquefaction to reach a mean of 312 mmol/kg (280-331) by 30 min at 37 degrees C. These results are at variance with the widely held view that semen osmolality is greater than that of serum, which results from its first being examined after liquefaction in vitro. Thus when sperm are routinely examined after liquefaction they have been subjected to osmotic stresses that are not experienced by spermatozoa entering the female tract at coitus.