During a long-term field study of a free-living population of California ground squirrels (Spermophilus beecheyi), blood samples were drawn at regular intervals from marked animals via femoral venipuncture, and plasma prolactin (PRL) was measured by homologous radioimmunoassay. Marked fluctuations with season and reproductive condition occurred in circulating PRL levels in both males and females, with peak levels occurring during the spring breeding season. Peak PRL levels in females were reached 4 weeks after peak male levels, but mean peak values did not differ significantly between the sexes. Among females, juveniles had lower mean PRL levels than adults, and yearlings had lower mean levels throughout their initial reproductive episodes than did older females.