The pertussis toxin-sensitive G proteins (guanine nucleotide binding proteins)-muscarinic receptor interactions in 18-day fetal to adult rat hearts were studied. The collective concentration of these G proteins was measured by pertussis toxin-dependent ADP-ribosylation and decreased from 1.5 pmol of [32P]ADP-ribose/mg of protein in the fetal heart to 0.5 pmol of [32P]ADP-ribose/mg of protein in 21-day postpartum and older animals in a nonlinear pattern. The muscarinic receptor density diminished 2-fold from 400 fmol/mg in 1-day-old neonate to 200 fmol/mg of protein in adult hearts in a linear manner. The receptor affinity for the agonist oxotremorine and the effect of guanine nucleotide on that affinity were monitored in heart preparations of the various ages. The IC50 value for oxotremorine/[3H]quinuclidinylbenzilate competition curves in the absence or presence of guanine nucleotide increased gradually with age. Modeling of the competition curves from 1-day-old neonate and adult preparations suggested the receptor has two oxotremorine affinity states in both age groups but the ability of guanine nucleotide to shift receptors from the higher affinity state increases with age. [3H] Oxotremorine binding to the higher affinity state had KD values of 95 and 170 pM in neonate and adult heart preparations. The concentration of 5'-guanylylimidodiphosphate which caused 50% displacement of [3H]oxotremorine binding was 1.3 and 0.22 microM in neonate and adult tissue. These results suggest that although the quantity of the G proteins and muscarinic receptors diminish with development, the sensitivity of the G protein:muscarinic receptor complex to guanine nucleotide increases.