The metabolic rates of different Ldh-B genotypes of developing Fundulus heteroclitus embryos were examined. Within 24 hours after fertilization, eggs homozygous for the southern (Ldh-Ba) allele began to exhibit significantly higher oxygen consumption than eggs homozygous for the northern (Ldh-Bb) allele. No significant differences existed between genotypes with respect to total LDH-B4 activity or ontogeny throughout development. Lactate concentrations were significantly different in the oocytes between genotypes (Ldh-Ba = 48 mM; Ldh-Bb = 40 mM). Furthermore, lactate was utilized at a high rate during early development. Lactate utilization was approximately 100 times higher than apparent glucose utilization, and both rates were significantly different between genotypes. Within 24 hours after fertilization, half of the lactate present in the oocyte was utilized in both genotypes. Oxygen consumption and heat dissipation measurements demonstrated that the Ldh-Ba homozygotes increased their metabolic rate dramatically between 10 and 15 hours after fertilization and that the Ldh-Bb homozygotes did not. It is suggested that lactate is a source of intermediates important for early development and that LDH-B4 may mediate the accumulation of that lactate in the oocyte and the utilization of lactate in the early development of the embryo. This metabolic role may lead to environmentally induced selection at the Ldh-B locus.