The Nb<sub>2</sub> rat lymphoma cell prolactin (PRL) bioassay was used, in conjunction with standard radioimmunoassay techniques, to examine the effects of various stressors on plasma bioactive (bio) and immunoreactive (ir) PRL levels in 3- to 5- and 22- to 24-month-old male Copenhagen-Fischer 344 rats. The animals were implanted with chronic intracardiac venous cannulas 24–48 h prior to experimentation. Serial blood samples were taken prior to, during and after exposure to either 15 min restraint stress, 15 min strobe light stress or 2 min ether stress. In 2 of 3 studies, basal prestress irPRL levels were significantly higher in old as compared to young male rats. However, in all studies, basal bioPRL levels were significantly lower in the older animals. Exposure to restraint, strobe light or ether stress induced significant and parallel increases in plasma ir- and bioPRL levels in young rats, and these stressors did not affect the ratio of blood bio/irPRL. Old rats exposed to the same stressors displayed similar increases in plasma irPRL, but bioPRL release was significantly attenuated and the ratio of plasma bio/irPRL levels was significantly lower when compared to young rats. In contrast, ether stress induced similar increases in plasma ir- and bioPRL levels in both age groups and restored the ratio of plasma bio/irPRL levels in old rats to that of young animals. These results demonstrate that, despite having significantly higher basal plasma irPRL levels, the bioactivity of this hormone is significantly diminished in old as compared to young male rats. These results also demonstrate that, although various stressors induce similar increases in irPRL, they have differential effects on the circulating levels of bioPRL in young and old rats. These findings suggest that during the aging process qualitative changes occur in PRL biosynthesis, processing and/or release, which significantly reduces the biopotency of this hormone when secreted during basal conditions and in response to certain types of stress.