In-medium effects and neutrino trapping on K+ and K- production and K- condensation in supeova matter are investigated in a chiral hadronic model. Our results show that neutrino trapping shifts the critical density for K- condensation to higher density, the Q values for K+ and K- production are not sensitive to neutrino trapping, in-medium effects decrease the Q values for NN → NNK+ K- and ∧N → NNK- and increase those for NN → N∧K+, K-p →∧π0 and K-n →∧π- as the density of supeova matter increases. Moreover,it is shown that neutrino trapping decreases the maximum masses of protoneutron stars compared with the neutrino-free case.