Luminal fluids were collected in vivo by micropuncture and cannulation from the rete testis, efferent ducts and ductus epididymidis of the rat to determine the composition of efferent duct fluids and the rates of reabsorption of water and solutes by the efferent ducts. The concentration of spermatozoa increased by a factor of about 25 from 2.42 x 10(4) microliters-1 in the fluid from the rete testis to 6.00 x 10(5) microliters-1 in fluid at the end of the efferent ducts, indicating that 96.2% of the fluid leaving the testis is reabsorbed from the lumen of the efferent ducts. Most of this reabsorption (70.9% or 33.4 microliters h-1) occurs in the region between the rete testis and the middle of the coni vasculosi, with only 25.1% (11.8 microliters h-1) occurring between the coni and the beginning of the ductus epididymidis. However, reabsorption across the epithelium occurs at about the same rate in both regions, with the proximal region reabsorbing 17.2 microliters cm-2 h-1 (70.9% of fluid entering the region) and the distal region reabsorbing 12.2 microliters cm-2 h-1 (86.1% of fluid entering the region). Consequently, the fluid reabsorption rate for the whole efferent duct system (15.6 microliters cm-2 h-1) is similar to the values for individual regions. The principal solutes in luminal fluids from the efferent ducts are Na+ (137-144 mM) and Cl- (113-130 mM). The estimated sum contribution of Na+, Cl- and K+ to the osmotic pressure of luminal fluids was approximately 80% at each site sampled in the efferent ducts. The osmotic pressure of luminal fluid samples (301-307 mosmol kg-1) did not vary significantly along the ducts or differ significantly from that of blood plasma. The results demonstrate that there is a net reabsorption in the efferent ducts of nearly all the testicular output of water and inorganic electrolytes, and most of the protein, and that, in comparison, the ductus epididymidis is a negligible site of net fluid reabsorption. The results indicate that the ductus epididymidis, rather than the efferent ducts, is the site of accumulation of high concentrations of specific organic compounds like inositol. The efferent ducts are similar to the homologous proximal tubules of the metanephric kidney in that the luminal electrolyte composition (principal solutes Na+ and Cl-) and osmotic pressure remain relatively stable and that fluid reabsorption is close to isotonic and occurs at the same rate as the reabsorption of Na+.