The object of this study was to examine whether prolonged‐release hard gelatin capsule formulations could be developed for dogs. Different viscosity grades of hydroxypropyl methylcellulose (HPMC) and sodium carboxymethylcellulose (NaCMC) were used to control drug release. Furosemide was chosen because of its wide use in the management of heart failure in dogs. Invitro, selecting different viscosity grades allowed good control of drug release, whereas in vivo the difference between formulations was clearly smaller. Although all formulations gave prolonged release, both inter‐ and intra‐individual variation in the plasma concentration‐time curves was high. It is difficult to develop prolonged‐release formulations for drugs such as furosemide with highly variable pharmacokinetic properties. However, hard gelatin capsules containing hydrophilic polymers could still be a suitable choice for s