Relaxin release from dispersed luteal cells was detected by a reverse hemolytic plaque assay to determine the influence of gestational age on basal relaxin secretion. Monodispersed luteal cells were derived by collagenase treatment of corpora lutea obtained from pigs in early (days 19-26), mid-(days 47-62), and late (days 80-99) gestation. The rate of plaque development under nonstimulated conditions progressively accelerated as gestation advanced, as did the rate of increase in plaque size. These results unequivocally demonstrate that basal relaxin release increases with advancing gestation. However, only about 50% of large luteal cells released relaxin at all stages of pregnancy examined up to day 100. These data indicated not only that basal relaxin release increases during pregnancy, but also that considerable heterogeneity exists with respect to relaxin output by individual cells. In contrast, both the basal rate of relaxin release and the percentage of cells committed to relaxin release declined significantly when luteal cells derived from preparturient sows (days 107-112 of gestation) were examined. Exposure of cultured luteal cells to prostaglandin F2 alpha (10(-8) and 10(-6) M) resulted in a rapid stimulation of relaxin secretion, but this agent did not recruit additional cells into the secretory pool. These data are consistent with the idea that autonomous changes and the action of secretagogues may combine at different times to achieve overall regulation of relaxin release by the corpus luteum. The significance of nonrelaxin-releasing luteal cells remains to be determined.