-I. The appearance of plasma [ 14C]glucose in the inferior cava vein after a pulse of 0.2 mmol of [U- 14C]L-alanine or [U-14C]glycerol/200 g body wt given through the portal vein was studied in fed 21 day pregnant rats and virgin controls under pentobarbital anesthesia. In both groups values were much higher when [U- 14C]glycerol was the administered tracer than when [U- 14C]L-alanine, and they were augmented in pregnant versus virgin animals at 1 min when receiving [U- 14C]glycerol and at 2 min when receiving [U- 14C]L-alanine. 2. 20 min after the tracers rats receiving (U- 14C]glycerol showed much higher liver ( 14C]g]ycogen and [ 14C]glyceride glycerol than those receiving [U- 14C]L-alanine. Radioactivity present in liver as [1 4C]glyceride glycerol was greater in pregnant than in virgin rats receiving [U- 14C]glycerol whereas radioactivity corresponding to [ 14C]fatty acids was lower in the former group receiving either tracer. 3. At 20 min after maternal treatments fetuses showed lower plasma [' 4 C]glycerol than [ 14C]alanine values but plasma [ 14C]glucose and liver [ 14C]glycogen values were much greater in fetuses from mothers receiving [U- 14C]glycerol than (U- 14C]L-alanine. 4. Besides showing the higher gluconeogenic efficiency in pregnant than in virgin rats, results indicate that at late gestation glycerol is used as a preferential substrate for both glucose and glyceride glycerol synthesis in liver.