This study examined the profile of S-GAGs synthesized by presecretory and secretory units isolated from rats at 17, 18 and 21 days in utero and 1, 6 and 35 days after birth. The units were incubated for 2 h in medium containing [35S]-sodium sulphate and then processed and analysed. Secretory units from 17-day embryonic presecretory units produced a S-GAG profile composed of approx. 73% chondroitin sulphate and 26% heparan sulphate. When cells of the embryonic units undergo cytodifferentiation to become secretory cells (18 days in utero), there is a major change in the relative amounts of S-GAG synthesized with 54% of the S-GAG produced being heparan sulphate and 41% chondroitin sulphate. There is a progressive increase in the relative amount of heparan sulphate produced and a concomitant decline in chondroitin sulphate as the secretory compartment of the gland matures. By 35 days after birth, the secretory units produced a S-GAG profile that was greater than 85% heparan sulphate and less than 10% chondroitin sulphate. The ratio of heparan sulphate/chondroitin sulphate production was 0.36 by 17-day embryonic presecretory units and shifted to 9.1 by 35-day postnatal units.