The effects of two new ergolines, pergolide and CH29-717, on prolactin secretion in vitro and in vivo were compared with those of bromocriptine. Both ergolines had in vitro potencies similar to that of bromocriptine, but had a shorter duration of action. However, in vivo both were longer acting than bromocriptine after a single dose of 100 micrograms, and effectively suppressed prolactin secretion for greater than 24 h. It would appear that the pharmacokinetics of these ergolines are different to those of bromocriptine, and this may have considerable therapeutic importance.