AbstractAfter oral doses of 30 mg of prazepam to humans, N‐descyclopropylmethylprazepam (desalkylprazepam, N‐desmethyldiazepam) is the only major drug‐related compound in plasma. Neither the parent drug, nor its major urinary metabolites were detected in plasma. The overall concentration‐time profile of desalkylprazepamDesalkylprazepam is used as a simplified name for N‐descyclopropylmethylprazepam.in the plasma of females was lower than, and significantly different (p0.05) from those in males (342ngml−1±60 S.D.). Concentrations declined in the plasma of either sex with similar half‐lives (mean 60 h, range 37–93 h). Apparent plasma desalkylprazepam clearances were also similar (mean 1.09 1 h−1, range 0.74–1.84 1 h−1).At 12 h after the last of multiple doses of prazepam (60 mg d−1for 3 days) to lactating women, mean plasma concentrations of desalkylprazepam were 823 ng ml−1±200 S.D. and declined with a mean half‐life of about 60 h over the time‐course studied. There was only slight uptake of desalkylprazepam into blood cells; plasma : whole blood concentration ratios were constant at about 1.6. Concentrations of desalkylprazepam in milk were low at about 10 percent of the corresponding plasma levels (e.g. 86 ng ml−1±37 S.D. at 12 h). The data suggest that, expressed on a mg kg−1basis, exposed neonates could receive about 4 per cent of the matern