Tubular secretion occurs simultaneously during re-absorption of filtrate.Tubular secretion occurs at Proximal Convoluted Tubule (PCT) and Distal Convoluted Tubule (D.C.T); for example, at proximal convoluted tubule, potassium is secreted by means of sodium-potassium pump, hydrogen ion is secreted by means of active transport and co-transport, i.e. anti-porter, and ammonia diffuses into renal tubule.Substances mainly secreted into renal tubule are; H+, K+, NH3, urea, Creatinine, histamine and drugs like penicillin.