This study examined the effect of increased BW on mean retention time (MRT) of both particulate and solute marker, gastrointestinal tract (GIT) development, and fiber digestion in the whole tract of growing Saanen goats using the slaughter technique. A total of 58 Saanen goats with initial BW of 15.7 ± 0.9 kg were allocated into 9 treatments with a 3 × 3 factorial arrangement consisting of 3 sexes (female, castrated males, and intact males) and 3 slaughter weights (initial, intermediate, and final; target BW of 16, 23, and 30 kg at slaughter, respectively). They were fed twice daily (0700 and 1600 h) with the identical diets for ad libitum intake. Mean retention time of particulate matter was estimated by in situ determination of indigestible NDF (iNDF), and the MRT of solute marker was determined by Cr-EDTA. Treatment effects were evaluated in a split-plot design, with sex as the main plot and slaughter weight as the subplot. Orthogonal polynomial contrasts were used to determine linear and quadratic effects of slaughter weight, whereas the effect of sex was compared using the Tukey test. The effects of sex and sex × slaughter weight were not significant for most of variables evaluated. The results showed that DMI (% BW) linearly decreased as slaughter weight increased ( < 0.01). Generally wet weight of the total GIT tissues (% BW) decreased and digesta pool sizes (g) linearly increased with increasing slaughter weight ( ≤ 0.05). The ratio of iNDF:NDF for both ingested diet and reticulorumen digesta linearly increased as slaughter weight increased ( ≤ 0.05). The MRT of particles did not change with increasing slaughter weight ( = 0.94). Mean retention time of particulate matter linearly increased in the omasum but linearly decreased in the abomasum with increasing slaughter weight ( < 0.01). Mean retention time of solute marker in the forestomachs linearly increased with increasing slaughter weight ( < 0.01). The results revealed a decreased selectivity with increasing BW, as supported by a greater ratio of iNDF:NDF for ingested diet. Increasing BW led to neither a longer particle MRT in the reticulorumen nor a digestive advantage. The results also indicated that, on average, 91% of fiber digestion occurred in the forestomachs of the goats.