Rat placental lactogen-I (rPL-I), the first prolactin-like hormone expressed in the placenta during pregnancy in the rat, is known to influence maternal functions. In the present study, we have investigated the effects of rPL-I on the growth and development of cultured whole rat embryos. Rat embryos, with or without ectoplacental cone (EPC) attached, were explanted at day 9 of gestation. After 48 h of culture, the embryos, enclosed by the yolk sacs, were assessed by the presence of visible heart contractions ('heart beats'), crown-rump length (CRL) and yolk sac diameter (YSD). When intact embryos with EPC were cultured, the concentrations of rPL-I and rPL-II (products of EPC) in the medium were 850+/-841 and 92+/-181 ng/ml respectively (means+/-s.e.m.). In embryo cultures with the EPC removed, rPL-I levels decreased to</=10 ng/ml, and only 70% of the embryos were viable, with visible heart beats. In the viable embryos, both CRL and embryonic DNA synthesis were reduced compared with controls, and the addition of rPL-I (1 microg/ml) did not prevent this reduction. YSD and yolk sac DNA synthesis were also reduced compared with control embryos, and the addition of rPL-I significantly prevented this decrease by 45%. In embryos cultured without EPC in the presence of neutralizing rabbit anti-rat prolactin serum (anti-rPRL), embryonic and yolk sac DNA synthesis were reduced by 35% compared with embryos exposed to normal rabbit serum. Addition of rPL-I significantly increased (P<0.05) embryonic and yolk sac growth. Thus the effects of rPL-I on embryo growth could only be seen in the absence of prolactin. The addition of human prolactin in the presence of anti-rPRL also resulted in significant increases (P<0.05) in embryonic DNA synthesis and CRL. These results suggest that rPL-I may substitute for prolactin to influence the growth of the rat embryo.