The effects of postnatal development, aging and the oestrous cycle on muscarinic acetylcholine receptor (mAChR) properties were examined in in vitro living slices of rat neocortex. Using the hydrophilic antagonist ([3H]NMS) to label cell surface mAChRs, an increase in both Bmax and Kd was found during the first postnatal weeks. These values peaked at between 20-40 days postnatally and then declined to adult levels. After 3 months of age, a steady decline in receptor number started: it was 10.1% lower at 10 months and 38.7% lower at 17 months of age. In contrast, Kd values increased, being 31.7 and 20% higher respectively at these ages. Carbachol-induced (4 h at 37 degrees C) down-regulation of receptor number was approximately 22.2% in newborn and 26.1% in adult (3-month-old) rats, but only 16.3% at 20-40 days of age. The degree of carbachol-induced down-regulation of mAChR was not affected in the older animals. Veratridine, which increases neural activity, also induced a significant reduction in [3H]NMS binding sites of 11.4% in rats aged 0-20 days and 22.4% in 3-month-old rats, but at 20-40 and 40-60 days of age no significant down-regulation of receptor number was observed. Furthermore, down-regulation was absent in the 10-month-old rats as well. Since a great variation in Bmax and Kd values was seen in 3-month-old females but not in male rats, we investigated mAChR characteristics during the oestrous cycle of female rats. In pro-oestrus, mACh receptor number was increased and affinity decreased in comparison with di-oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)